feat(maestro): port layout system (tabs, splits, multiview) to React

Closes #249: layoutStore.ts wraps the existing layout.ts tree model
in Zustand, persisting to localStorage on a 400ms debounce and adding
boot reconciliation to prune leaves for sessions no longer live.
MultiView/TabBar/PaneContainer/PaneTitle render tabs, nested splits
with draggable dividers, drag-and-drop tab reordering, and dispatch
each pane to TerminalPane or StructuredPane by session view.

Preview mode (#251) and sidebar drag-to-dock stay out of scope here.

web/src/components/layout/MultiView.test.tsx

@@ -0,0 +1,96 @@
+// @vitest-environment happy-dom
+import { cleanup, fireEvent, render, screen } from "@testing-library/react";
+import { afterEach, describe, expect, it, vi } from "vitest";
+
+import { leaves } from "../../layout.js";
+import { createLayoutStore } from "../../stores/layoutStore.js";
+import type { Session } from "../../types.js";
+import { MultiView } from "./MultiView.js";
+
+vi.mock("../terminal/TerminalPane.js", () => ({
+  TerminalPane: ({ sessionId }: { sessionId: string }) => <div data-testid={`terminal-${sessionId}`} />,
+}));
+
+vi.mock("../structured/StructuredPane.js", () => ({
+  StructuredPane: ({ sessionId }: { sessionId: string }) => <div data-testid={`structured-${sessionId}`} />,
+}));
+
+afterEach(cleanup);
+
+function store() {
+  return createLayoutStore({
+    load: () => null,
+    save: vi.fn(),
+    scheduler: { setTimer: () => 1, clearTimer: () => {} },
+  });
+}
+
+function session(id: string, over: Partial<Session> = {}): Session {
+  return {
+    id,
+    cwd: "/home/n",
+    command: "claude",
+    env: {},
+    metadata: {},
+    status: "Running",
+    created_at: "2026-01-01T00:00:00Z",
+    ...over,
+  } as Session;
+}
+
+describe("MultiView", () => {
+  it("shows the empty state when the active tab has no panes", () => {
+    const s = store();
+    render(<MultiView store={s} />);
+    expect(screen.getByText(/No session open/)).toBeTruthy();
+  });
+
+  it("renders a pane per leaf, dispatching to the terminal or structured body", () => {
+    const s = store();
+    s.getState().openSession("s1");
+    s.getState().openSessionBeside("s2");
+    const sessions = new Map([
+      ["s1", session("s1")],
+      ["s2", session("s2", { view: "Structured" })],
+    ]);
+    render(<MultiView store={s} sessions={sessions} />);
+    expect(screen.getByTestId("terminal-s1")).toBeTruthy();
+    expect(screen.getByTestId("structured-s2")).toBeTruthy();
+  });
+
+  it("split right adds a second pane to the tree", () => {
+    const s = store();
+    s.getState().openSession("s1");
+    render(<MultiView store={s} sessions={new Map([["s1", session("s1")]])} />);
+    fireEvent.click(screen.getByTitle("Split right"));
+    expect(screen.getAllByText("Empty pane")).toHaveLength(1);
+  });
+
+  it("close pane removes it", () => {
+    const s = store();
+    s.getState().openSession("s1");
+    const leafId = leaves(s.getState().layout.tabs[0].root)[0].id;
+    s.getState().splitFocused(leafId, "row"); // adds an empty leaf beside s1
+    render(<MultiView store={s} sessions={new Map([["s1", session("s1")]])} />);
+    // splitFocused(after: true) puts the fresh empty leaf second — close it.
+    fireEvent.click(screen.getAllByTitle("Close pane")[1]);
+    expect(screen.queryByText("Empty pane")).toBeNull();
+    expect(screen.getByTestId("terminal-s1")).toBeTruthy();
+  });
+
+  it("prunes a leaf whose session no longer exists, once the live set is known", () => {
+    const s = store();
+    s.getState().openSession("s1");
+    s.getState().openSessionBeside("dead");
+    render(<MultiView store={s} sessions={new Map([["s1", session("s1")]])} />);
+    expect(screen.getByTestId("terminal-s1")).toBeTruthy();
+    expect(screen.queryByTestId("terminal-dead")).toBeNull();
+  });
+
+  it("does not touch the layout while the live set is still unknown", () => {
+    const s = store();
+    s.getState().openSession("s1");
+    render(<MultiView store={s} />);
+    expect(screen.getByTestId("terminal-s1")).toBeTruthy();
+  });
+});

web/src/components/layout/MultiView.tsx

@@ -0,0 +1,203 @@
+// React port of multiview.ts's tab + split tree (issue #249). Preview mode
+// (docs/PREVIEW.md) is issue #251; sidebar drag-to-dock and the mobile
+// single-pane collapse stay with whichever issue wires this into the shell
+// (#253) — this renders the tree itself: tabs, nested splits, pane chrome.
+
+import type { ReactNode } from "react";
+import { useEffect, useRef } from "react";
+import { useStore } from "zustand";
+
+import { activeTab, type Dir, type Node } from "../../layout.js";
+import type { LayoutStore, LayoutStoreApi } from "../../stores/layoutStore.js";
+import type { Session, Uuid } from "../../types.js";
+import { PaneContainer } from "./PaneContainer.js";
+import { PaneTitle } from "./PaneTitle.js";
+import { TabBar } from "./TabBar.js";
+
+export interface MultiViewProps {
+  store: LayoutStoreApi;
+  /** Live sessions, keyed by id — drives pane titles and (once known) boot
+   *  reconciliation. Passing `undefined` defers reconciliation. */
+  sessions?: ReadonlyMap<Uuid, Session>;
+  onDisconnect?: (sessionId: Uuid) => void;
+  emptyState?: ReactNode;
+}
+
+export function MultiView({ store, sessions, onDisconnect, emptyState }: MultiViewProps) {
+  const layout = useStore(store, (s: LayoutStore) => s.layout);
+  const tab = activeTab(layout);
+
+  const reconciledRef = useRef(false);
+  useEffect(() => {
+    if (reconciledRef.current || !sessions) return;
+    reconciledRef.current = true;
+    store.getState().reconcile(new Set(sessions.keys()));
+  }, [sessions, store]);
+
+  return (
+    <div className="mv flex h-full min-h-0 flex-col">
+      <TabBar store={store} />
+      <div className="mv-canvas min-h-0 flex-1">
+        {tab.root ? (
+          <SplitNode node={tab.root} tab={tab} store={store} sessions={sessions} onDisconnect={onDisconnect} />
+        ) : (
+          (emptyState ?? (
+            <div className="mv-empty flex h-full items-center justify-center p-sp-4 text-fs-13 text-faint">
+              No session open — pick one from the sidebar, or spawn a new one.
+            </div>
+          ))
+        )}
+      </div>
+    </div>
+  );
+}
+
+interface TreeProps {
+  tab: { id: string; focusedLeafId: string | null };
+  store: LayoutStoreApi;
+  sessions?: ReadonlyMap<Uuid, Session>;
+  onDisconnect?: (sessionId: Uuid) => void;
+}
+
+function SplitNode({ node, ...rest }: TreeProps & { node: Node }) {
+  if (node.kind === "leaf") {
+    return <Pane leaf={node} isFocused={rest.tab.focusedLeafId === node.id} {...rest} />;
+  }
+  return <SplitContainer node={node} {...rest} />;
+}
+
+function SplitContainer({ node, ...rest }: TreeProps & { node: Extract<Node, { kind: "split" }> }) {
+  const slotA = useRef<HTMLDivElement | null>(null);
+  const slotB = useRef<HTMLDivElement | null>(null);
+  const containerRef = useRef<HTMLDivElement | null>(null);
+
+  const onDividerPointerDown = (e: React.PointerEvent<HTMLDivElement>) => {
+    e.preventDefault();
+    const divider = e.currentTarget;
+    const container = containerRef.current;
+    if (!container) return;
+    divider.setPointerCapture(e.pointerId);
+    const rect = container.getBoundingClientRect();
+    const move = (ev: PointerEvent) => {
+      const frac =
+        node.dir === "row" ? (ev.clientX - rect.left) / rect.width : (ev.clientY - rect.top) / rect.height;
+      node.ratio = Math.max(0.1, Math.min(0.9, frac));
+      if (slotA.current) slotA.current.style.flexGrow = String(node.ratio);
+      if (slotB.current) slotB.current.style.flexGrow = String(1 - node.ratio);
+    };
+    const up = (ev: PointerEvent) => {
+      try {
+        divider.releasePointerCapture(ev.pointerId);
+      } catch {
+        // already released
+      }
+      divider.removeEventListener("pointermove", move);
+      divider.removeEventListener("pointerup", up);
+      rest.store.getState().commitLayout();
+    };
+    divider.addEventListener("pointermove", move);
+    divider.addEventListener("pointerup", up);
+  };
+
+  return (
+    <div
+      ref={containerRef}
+      className={`mv-split flex h-full min-h-0 w-full ${node.dir === "row" ? "flex-row" : "flex-col"}`}
+    >
+      <div ref={slotA} className="mv-slot min-h-0 min-w-0" style={{ flex: `${node.ratio} 1 0` }}>
+        <SplitNode node={node.a} {...rest} />
+      </div>
+      <div
+        className={
+          "mv-divider flex-none bg-border-strong hover:bg-accent " +
+          (node.dir === "row" ? "w-[3px] cursor-col-resize" : "h-[3px] cursor-row-resize")
+        }
+        onPointerDown={onDividerPointerDown}
+      />
+      <div ref={slotB} className="mv-slot min-h-0 min-w-0" style={{ flex: `${1 - node.ratio} 1 0` }}>
+        <SplitNode node={node.b} {...rest} />
+      </div>
+    </div>
+  );
+}
+
+function Pane({
+  leaf,
+  isFocused,
+  store,
+  sessions,
+  onDisconnect,
+}: TreeProps & { leaf: Extract<Node, { kind: "leaf" }>; isFocused: boolean }) {
+  const session = leaf.sessionId ? sessions?.get(leaf.sessionId) : undefined;
+  const splitFocused = (dir: Dir) => store.getState().splitFocused(leaf.id, dir);
+
+  return (
+    <div
+      className={
+        "mv-pane flex h-full min-h-0 min-w-0 flex-col border " +
+        (isFocused ? "border-accent" : "border-border-strong")
+      }
+      data-leaf-id={leaf.id}
+      data-session-id={leaf.sessionId ?? undefined}
+      onPointerDownCapture={() => store.getState().focusLeaf(leaf.id)}
+    >
+      <div className="mv-pane-header flex items-center justify-between gap-sp-2 border-b border-border-strong bg-surface px-sp-2 py-sp-1 text-fs-12">
+        {leaf.sessionId ? (
+          <PaneTitle sessionId={leaf.sessionId} session={session} />
+        ) : (
+          <span className="text-faint">Empty pane</span>
+        )}
+        <div className="mv-pane-tools flex items-center gap-sp-1">
+          {leaf.sessionId && onDisconnect && (
+            <button
+              type="button"
+              className="mv-tool rounded-r-1 px-sp-1 text-faint hover:bg-surface-3 hover:text-ink"
+              title="Disconnect"
+              onClick={() => onDisconnect(leaf.sessionId!)}
+            >
+              ⏻
+            </button>
+          )}
+          <button
+            type="button"
+            className="mv-tool rounded-r-1 px-sp-1 text-faint hover:bg-surface-3 hover:text-ink"
+            title="Split right"
+            onClick={() => splitFocused("row")}
+          >
+            ▥
+          </button>
+          <button
+            type="button"
+            className="mv-tool rounded-r-1 px-sp-1 text-faint hover:bg-surface-3 hover:text-ink"
+            title="Split down"
+            onClick={() => splitFocused("col")}
+          >
+            ▤
+          </button>
+          <button
+            type="button"
+            className="mv-tool rounded-r-1 px-sp-1 text-faint hover:bg-surface-3 hover:text-ink"
+            title="Close pane"
+            onClick={() => store.getState().closePane(leaf.id)}
+          >
+            ✕
+          </button>
+        </div>
+      </div>
+      <div className="mv-pane-host min-h-0 flex-1">
+        {leaf.sessionId ? (
+          <PaneContainer
+            sessionId={leaf.sessionId}
+            session={session}
+            focus={isFocused}
+            onClosed={() => store.getState().closeSession(leaf.sessionId!)}
+          />
+        ) : (
+          <div className="mv-pane-empty flex h-full items-center justify-center p-sp-4 text-fs-13 text-faint">
+            Empty pane — pick a session from the sidebar, or spawn one.
+          </div>
+        )}
+      </div>
+    </div>
+  );
+}

web/src/components/layout/PaneContainer.test.tsx

@@ -0,0 +1,54 @@
+// @vitest-environment happy-dom
+import { cleanup, render, screen } from "@testing-library/react";
+import { afterEach, describe, expect, it, vi } from "vitest";
+
+import type { Session } from "../../types.js";
+import { PaneContainer } from "./PaneContainer.js";
+
+vi.mock("../terminal/TerminalPane.js", () => ({
+  TerminalPane: ({ sessionId, focus }: { sessionId: string; focus?: boolean }) => (
+    <div data-testid="terminal-pane" data-session-id={sessionId} data-focus={String(focus)} />
+  ),
+}));
+
+vi.mock("../structured/StructuredPane.js", () => ({
+  StructuredPane: ({ sessionId, focus }: { sessionId: string; focus?: boolean }) => (
+    <div data-testid="structured-pane" data-session-id={sessionId} data-focus={String(focus)} />
+  ),
+}));
+
+afterEach(cleanup);
+
+function session(over: Partial<Session> = {}): Session {
+  return {
+    id: "s1",
+    cwd: "/home/n",
+    command: "claude",
+    env: {},
+    metadata: {},
+    status: "Running",
+    created_at: "2026-01-01T00:00:00Z",
+    ...over,
+  } as Session;
+}
+
+describe("PaneContainer", () => {
+  it("renders TerminalPane when the session has no view (defaults to Terminal)", () => {
+    render(<PaneContainer sessionId="s1" session={session()} />);
+    expect(screen.getByTestId("terminal-pane")).toBeTruthy();
+    expect(screen.queryByTestId("structured-pane")).toBeNull();
+  });
+
+  it("renders TerminalPane when the session isn't known yet", () => {
+    render(<PaneContainer sessionId="s1" />);
+    expect(screen.getByTestId("terminal-pane")).toBeTruthy();
+  });
+
+  it("renders StructuredPane for a Structured session", () => {
+    render(<PaneContainer sessionId="s1" session={session({ view: "Structured" })} focus={false} />);
+    const pane = screen.getByTestId("structured-pane");
+    expect(pane.dataset.sessionId).toBe("s1");
+    expect(pane.dataset.focus).toBe("false");
+    expect(screen.queryByTestId("terminal-pane")).toBeNull();
+  });
+});

web/src/components/layout/PaneContainer.tsx

@@ -0,0 +1,49 @@
+// Dispatches a leaf's session to the right body — TerminalPane for a PTY
+// session, StructuredPane for an ACP one (session.view, #133/#140/#249).
+
+import { forwardRef, useImperativeHandle, useRef } from "react";
+
+import type { Session, Uuid } from "../../types.js";
+import { StructuredPane, type StructuredPaneHandle } from "../structured/StructuredPane.js";
+import { TerminalPane, type TerminalPaneHandle } from "../terminal/TerminalPane.js";
+
+export interface PaneContainerHandle {
+  focusTerminal(): void;
+  /** No-op for a structured pane — it has no PTY (pane.ts's PaneConnection contract). */
+  sendInput(data: string): void;
+}
+
+export interface PaneContainerProps {
+  sessionId: Uuid;
+  session?: Session;
+  /** Grabs the keyboard once connected. A sidebar preview passes `false`. */
+  focus?: boolean;
+  onClosed?: () => void;
+  className?: string;
+}
+
+export const PaneContainer = forwardRef<PaneContainerHandle, PaneContainerProps>(function PaneContainer(
+  { sessionId, session, focus, onClosed, className },
+  ref,
+) {
+  const terminalRef = useRef<TerminalPaneHandle>(null);
+  const structuredRef = useRef<StructuredPaneHandle>(null);
+
+  useImperativeHandle(
+    ref,
+    () => ({
+      focusTerminal: () => {
+        terminalRef.current?.focusTerminal();
+        structuredRef.current?.focusComposer();
+      },
+      sendInput: (data: string) => terminalRef.current?.sendInput(data),
+    }),
+    [],
+  );
+
+  return session?.view === "Structured" ? (
+    <StructuredPane ref={structuredRef} sessionId={sessionId} focus={focus} onClosed={onClosed} className={className} />
+  ) : (
+    <TerminalPane ref={terminalRef} sessionId={sessionId} focus={focus} onClosed={onClosed} className={className} />
+  );
+});

web/src/components/layout/PaneTitle.test.tsx

@@ -0,0 +1,48 @@
+// @vitest-environment happy-dom
+import { cleanup, render, screen } from "@testing-library/react";
+import { afterEach, describe, expect, it } from "vitest";
+
+import type { Session } from "../../types.js";
+import { PaneTitle } from "./PaneTitle.js";
+
+afterEach(cleanup);
+
+function session(over: Partial<Session> = {}): Session {
+  return {
+    id: "s1",
+    cwd: "/home/n",
+    command: "claude",
+    env: {},
+    metadata: {},
+    status: "Running",
+    created_at: "2026-01-01T00:00:00Z",
+    ...over,
+  } as Session;
+}
+
+describe("PaneTitle", () => {
+  it("falls back to a short session id when the session isn't known yet", () => {
+    render(<PaneTitle sessionId="abcdef12-3456-7890-abcd-ef1234567890" />);
+    expect(screen.getByText(/abcdef12/)).toBeTruthy();
+  });
+
+  it("shows the name alone when the command matches it", () => {
+    render(<PaneTitle sessionId="s1" session={session({ command: "claude", metadata: { name: "claude" } })} />);
+    expect(screen.getByText("claude")).toBeTruthy();
+    expect(document.querySelector(".at-cmd")).toBeNull();
+  });
+
+  it("shows the command alongside a custom name", () => {
+    render(
+      <PaneTitle sessionId="s1" session={session({ command: "hx src/main.rs", metadata: { name: "editor" } })} />,
+    );
+    expect(screen.getByText("editor")).toBeTruthy();
+    expect(document.querySelector(".at-cmd")).not.toBeNull();
+  });
+
+  it("never injects markup from a session name", () => {
+    render(<PaneTitle sessionId="s1" session={session({ metadata: { name: "<img onerror=x>" } })} />);
+    expect(document.querySelector("img")).toBeNull();
+    expect(screen.getByText("<img onerror=x>")).toBeTruthy();
+  });
+});

web/src/components/layout/PaneTitle.tsx

@@ -0,0 +1,39 @@
+// React port of paneTitle.ts: a pane header's session identity — the name,
+// then the command as its tool mark when recognised, same as the sidebar
+// card's own CommandFragment (SessionCard.tsx). JSX escapes text nodes, so
+// unlike the vanilla string-builder this needs no manual HTML escaping.
+
+import { splitCommand, toolIconSvg } from "../../toolIcon.js";
+import { sessionName, type Session, type Uuid } from "../../types.js";
+
+export interface PaneTitleProps {
+  sessionId: Uuid;
+  session?: Session;
+  className?: string;
+}
+
+export function PaneTitle({ sessionId, session, className }: PaneTitleProps) {
+  if (!session) {
+    return (
+      <span className={className ?? "at-title"} title={sessionId}>
+        session <span className="sid">{sessionId.slice(0, 8)}</span>
+      </span>
+    );
+  }
+
+  const name = sessionName(session);
+  const showCommand = Boolean(session.command) && session.command !== name;
+  const { tool, program } = splitCommand(session.command);
+  const icon = tool ? toolIconSvg(tool) : null;
+
+  return (
+    <span className={className ?? "at-title"} title={`${name} · ${session.command}  (${sessionId})`}>
+      <span className="at-name">{name}</span>
+      {showCommand && (
+        <span className="at-cmd">
+          {icon ? <span className="contents" dangerouslySetInnerHTML={{ __html: icon }} /> : <span>{program}</span>}
+        </span>
+      )}
+    </span>
+  );
+}

web/src/components/layout/TabBar.test.tsx

@@ -0,0 +1,88 @@
+// @vitest-environment happy-dom
+import { cleanup, fireEvent, render, screen } from "@testing-library/react";
+import { afterEach, describe, expect, it, vi } from "vitest";
+
+import { createLayoutStore } from "../../stores/layoutStore.js";
+import { TabBar } from "./TabBar.js";
+
+afterEach(cleanup);
+
+function store() {
+  return createLayoutStore({
+    load: () => null,
+    save: vi.fn(),
+    scheduler: { setTimer: () => 1, clearTimer: () => {} },
+  });
+}
+
+function dataTransfer() {
+  const data = new Map<string, string>();
+  return {
+    setData: (type: string, value: string) => data.set(type, value),
+    getData: (type: string) => data.get(type) ?? "",
+    effectAllowed: "",
+  };
+}
+
+describe("TabBar", () => {
+  it("renders one tab per layout tab, marking the active one", () => {
+    const s = store();
+    s.getState().addTab();
+    render(<TabBar store={s} />);
+    const tabs = screen.getAllByRole("tab");
+    expect(tabs).toHaveLength(2);
+    expect(tabs[1].getAttribute("aria-selected")).toBe("true");
+    expect(tabs[0].getAttribute("aria-selected")).toBe("false");
+  });
+
+  it("shows the open-session count next to a tab's title", () => {
+    const s = store();
+    s.getState().openSession("s1");
+    render(<TabBar store={s} />);
+    expect(screen.getByText(/· 1/)).toBeTruthy();
+  });
+
+  it("clicking a tab selects it", () => {
+    const s = store();
+    s.getState().addTab();
+    const [first] = s.getState().layout.tabs;
+    render(<TabBar store={s} />);
+    fireEvent.click(screen.getAllByRole("tab")[0]);
+    expect(s.getState().layout.activeTabId).toBe(first.id);
+  });
+
+  it("the + button adds a tab", () => {
+    const s = store();
+    render(<TabBar store={s} />);
+    fireEvent.click(screen.getByTitle("New tab"));
+    expect(s.getState().layout.tabs).toHaveLength(2);
+  });
+
+  it("hides the close control on the last remaining tab", () => {
+    const s = store();
+    render(<TabBar store={s} />);
+    expect(screen.queryByTitle("Close tab")).toBeNull();
+  });
+
+  it("closing a tab removes it", () => {
+    const s = store();
+    s.getState().addTab();
+    render(<TabBar store={s} />);
+    fireEvent.click(screen.getAllByTitle("Close tab")[0]);
+    expect(s.getState().layout.tabs).toHaveLength(1);
+  });
+
+  it("dragging a tab onto another reorders them", () => {
+    const s = store();
+    s.getState().addTab();
+    s.getState().addTab();
+    const [a, b, c] = s.getState().layout.tabs;
+    render(<TabBar store={s} />);
+    const tabs = screen.getAllByRole("tab");
+    const dt = dataTransfer();
+    fireEvent.dragStart(tabs[0], { dataTransfer: dt });
+    fireEvent.dragOver(tabs[2], { dataTransfer: dt });
+    fireEvent.drop(tabs[2], { dataTransfer: dt });
+    expect(s.getState().layout.tabs.map((t) => t.id)).toEqual([b.id, c.id, a.id]);
+  });
+});

web/src/components/layout/TabBar.tsx

@@ -0,0 +1,91 @@
+// The tab strip above the canvas (issue #249): click to switch, drag to
+// reorder, a close ✕ per tab (hidden on the last one), a trailing + to add.
+
+import { useState } from "react";
+import { useStore } from "zustand";
+
+import { leaves } from "../../layout.js";
+import type { LayoutStore, LayoutStoreApi } from "../../stores/layoutStore.js";
+
+export interface TabBarProps {
+  store: LayoutStoreApi;
+}
+
+export function TabBar({ store }: TabBarProps) {
+  const layout = useStore(store, (s: LayoutStore) => s.layout);
+  const [draggingId, setDraggingId] = useState<string | null>(null);
+  const [overId, setOverId] = useState<string | null>(null);
+
+  return (
+    <div className="mv-tabs flex items-center gap-sp-1 overflow-x-auto border-b border-border-strong bg-surface px-sp-2 text-fs-12" role="tablist">
+      {layout.tabs.map((tab) => {
+        const count = leaves(tab.root).filter((l) => l.sessionId).length;
+        const isActive = tab.id === layout.activeTabId;
+        const isDropTarget = overId === tab.id && draggingId !== null && draggingId !== tab.id;
+        return (
+          <button
+            key={tab.id}
+            type="button"
+            role="tab"
+            aria-selected={isActive}
+            draggable
+            data-tab-id={tab.id}
+            className={
+              "mv-tab flex items-center gap-sp-1 rounded-t-r-1 px-sp-2 py-sp-1 " +
+              (isActive ? "bg-surface-2 text-ink" : "text-faint hover:text-ink") +
+              (isDropTarget ? " outline outline-2 outline-accent" : "")
+            }
+            onDragStart={(e) => {
+              setDraggingId(tab.id);
+              e.dataTransfer.setData("text/plain", tab.id);
+              e.dataTransfer.effectAllowed = "move";
+            }}
+            onDragOver={(e) => {
+              if (!draggingId || draggingId === tab.id) return;
+              e.preventDefault();
+              setOverId(tab.id);
+            }}
+            onDragLeave={() => setOverId((current) => (current === tab.id ? null : current))}
+            onDrop={(e) => {
+              e.preventDefault();
+              const fromId = e.dataTransfer.getData("text/plain") || draggingId;
+              setDraggingId(null);
+              setOverId(null);
+              if (fromId && fromId !== tab.id) store.getState().reorderTab(fromId, tab.id);
+            }}
+            onDragEnd={() => {
+              setDraggingId(null);
+              setOverId(null);
+            }}
+            onClick={() => store.getState().selectTab(tab.id)}
+          >
+            <span>
+              {tab.title}
+              {count ? ` · ${count}` : ""}
+            </span>
+            {layout.tabs.length > 1 && (
+              <span
+                className="mv-tab-close text-faint hover:text-ink"
+                title="Close tab"
+                onClick={(e) => {
+                  e.stopPropagation();
+                  store.getState().closeTab(tab.id);
+                }}
+              >
+                ✕
+              </span>
+            )}
+          </button>
+        );
+      })}
+      <button
+        type="button"
+        className="mv-tab-add px-sp-2 py-sp-1 text-muted hover:text-ink"
+        title="New tab"
+        onClick={() => store.getState().addTab()}
+      >
+        +
+      </button>
+    </div>
+  );
+}

web/src/components/spawn/BranchPicker.test.tsx

@@ -0,0 +1,79 @@
+// @vitest-environment happy-dom
+import { cleanup, fireEvent, render, screen } from "@testing-library/react";
+import { afterEach, describe, expect, it, vi } from "vitest";
+
+import type { BranchInfo } from "../../types.js";
+import { BranchPicker, type BranchPickerProps } from "./BranchPicker.js";
+
+afterEach(cleanup);
+
+function props(over: Partial<BranchPickerProps> = {}): BranchPickerProps {
+  return {
+    enabled: true,
+    onToggleEnabled: vi.fn(),
+    hint: null,
+    baseRepo: "/repo",
+    mode: "new",
+    onModeChange: vi.fn(),
+    newBranch: "",
+    onNewBranchChange: vi.fn(),
+    branches: undefined,
+    existingBranch: "",
+    onExistingBranchChange: vi.fn(),
+    ...over,
+  };
+}
+
+describe("BranchPicker", () => {
+  it("shows the hint instead of the body while unresolved", () => {
+    render(<BranchPicker {...props({ hint: "Choose a working directory first." })} />);
+    expect(screen.getByText("Choose a working directory first.")).not.toBeNull();
+    expect(screen.queryByText("New branch name")).toBeNull();
+  });
+
+  it("shows the new-branch field by default and the repo line once resolved", () => {
+    render(<BranchPicker {...props()} />);
+    expect(screen.getByText("repo: /repo")).not.toBeNull();
+    expect(screen.getByPlaceholderText("feat/foo")).not.toBeNull();
+    expect(screen.queryByText("Pick a branch")).toBeNull();
+  });
+
+  it("switches to the existing-branch select and reports the mode change", () => {
+    const onModeChange = vi.fn();
+    render(<BranchPicker {...props({ onModeChange })} />);
+
+    fireEvent.click(screen.getByLabelText("Existing branch"));
+
+    expect(onModeChange).toHaveBeenCalledWith("existing");
+  });
+
+  it("disables a branch already held by another worktree, and the main checkout's HEAD", () => {
+    const branches: BranchInfo[] = [
+      { name: "main", is_head: true },
+      { name: "feat/a", is_head: false, worktree: "/repos/a" },
+      { name: "feat/b", is_head: false },
+    ];
+    render(<BranchPicker {...props({ mode: "existing", branches })} />);
+
+    const main = screen.getByRole("option", { name: /main — held by the main checkout/ }) as HTMLOptionElement;
+    const a = screen.getByRole("option", { name: /feat\/a — held by \/repos\/a/ }) as HTMLOptionElement;
+    const b = screen.getByRole("option", { name: "feat/b" }) as HTMLOptionElement;
+    expect(main.disabled).toBe(true);
+    expect(a.disabled).toBe(true);
+    expect(b.disabled).toBe(false);
+  });
+
+  it("shows a placeholder option when no local branches were found", () => {
+    render(<BranchPicker {...props({ mode: "existing", branches: [] })} />);
+    expect(screen.getByText("No local branches found.")).not.toBeNull();
+  });
+
+  it("toggles the worktree checkbox", () => {
+    const onToggleEnabled = vi.fn();
+    render(<BranchPicker {...props({ onToggleEnabled })} />);
+
+    fireEvent.click(screen.getByLabelText("Create git worktree"));
+
+    expect(onToggleEnabled).toHaveBeenCalledWith(false);
+  });
+});

web/src/components/spawn/BranchPicker.tsx

@@ -0,0 +1,119 @@
+// The spawn form's worktree section (#61): create-worktree toggle, then
+// (once a base repo is known) new-branch-vs-existing-branch, mirroring
+// spawnForm.ts's `.adv-group`/`.wt-body`/`.wt-mode` markup.
+
+import type { BranchInfo } from "../../types.js";
+import type { WorktreeBranchMode } from "../../spawnForm.js";
+
+export interface BranchPickerProps {
+  enabled: boolean;
+  onToggleEnabled(enabled: boolean): void;
+  /** Status line shown in place of the body — "Checking…", "not a repo",
+   * "choose a directory first". `null` once the body itself can render. */
+  hint: string | null;
+  /** The repo a worktree would branch from, once resolved. */
+  baseRepo: string | null;
+  mode: WorktreeBranchMode;
+  onModeChange(mode: WorktreeBranchMode): void;
+  newBranch: string;
+  onNewBranchChange(value: string): void;
+  /** This cwd's local branches, or `undefined` while unknown/not yet loaded. */
+  branches: BranchInfo[] | undefined;
+  existingBranch: string;
+  onExistingBranchChange(value: string): void;
+}
+
+export function BranchPicker(props: BranchPickerProps) {
+  return (
+    <div
+      className={`flex flex-col gap-sp-3 border-l-2 pl-sp-3 ${
+        props.enabled ? "border-accent" : "border-border"
+      }`}
+    >
+      <label className="flex cursor-pointer items-center gap-sp-2 text-fs-14 text-ink">
+        <input
+          type="checkbox"
+          data-role="toggle"
+          className="size-[18px] accent-accent"
+          checked={props.enabled}
+          onChange={(e) => props.onToggleEnabled(e.currentTarget.checked)}
+        />
+        Create git worktree
+      </label>
+
+      {props.hint && <p className="m-0 text-fs-12 text-muted">{props.hint}</p>}
+
+      {props.enabled && !props.hint && (
+        <div className="flex flex-col gap-sp-3">
+          {props.baseRepo && (
+            <p className="m-0 font-mono text-fs-12 text-muted">repo: {props.baseRepo}</p>
+          )}
+          <div role="radiogroup" aria-label="Branch source" className="flex gap-sp-4">
+            <label className="flex cursor-pointer items-center gap-sp-2 text-fs-14 text-ink">
+              <input
+                type="radio"
+                name="worktree_branch_mode"
+                checked={props.mode === "new"}
+                onChange={() => props.onModeChange("new")}
+              />
+              New branch
+            </label>
+            <label className="flex cursor-pointer items-center gap-sp-2 text-fs-14 text-ink">
+              <input
+                type="radio"
+                name="worktree_branch_mode"
+                checked={props.mode === "existing"}
+                onChange={() => props.onModeChange("existing")}
+              />
+              Existing branch
+            </label>
+          </div>
+
+          {props.mode === "new" ? (
+            <label className="flex flex-col gap-1 text-fs-12 text-muted">
+              New branch name
+              <input
+                name="worktree_branch"
+                type="text"
+                autoComplete="off"
+                placeholder="feat/foo"
+                className="w-full rounded-r-2 border border-border bg-surface-2 px-sp-3 py-2 font-mono text-fs-15 text-ink"
+                value={props.newBranch}
+                onChange={(e) => props.onNewBranchChange(e.currentTarget.value)}
+              />
+            </label>
+          ) : (
+            <label className="flex flex-col gap-1 text-fs-12 text-muted">
+              Pick a branch
+              <select
+                className="w-full rounded-r-2 border border-border bg-surface-2 px-sp-3 py-2 font-mono text-fs-15 text-ink"
+                value={props.existingBranch}
+                onChange={(e) => props.onExistingBranchChange(e.currentTarget.value)}
+              >
+                {branchOptions(props.branches)}
+              </select>
+            </label>
+          )}
+        </div>
+      )}
+    </div>
+  );
+}
+
+function branchOptions(branches: BranchInfo[] | undefined) {
+  if (!branches || branches.length === 0) {
+    return <option value="">No local branches found.</option>;
+  }
+  return branches.map((b) => {
+    // A branch already checked out anywhere stays visible but unpickable —
+    // git refuses a second checkout. That includes the main checkout's own
+    // HEAD, which is commonly `main` and would otherwise be the default pick,
+    // making every existing-branch spawn fail (#54 review).
+    const held = b.worktree ?? (b.is_head ? "the main checkout" : null);
+    return (
+      <option key={b.name} value={b.name} disabled={held !== null} title={held ?? ""}>
+        {held ? `${b.name} — held by ${held}` : b.name}
+      </option>
+    );
+  });
+}

web/src/components/spawn/DirAutocomplete.test.tsx

@@ -0,0 +1,109 @@
+// @vitest-environment happy-dom
+import { act, cleanup, fireEvent, render, screen, waitFor } from "@testing-library/react";
+import { useState } from "react";
+import { afterEach, describe, expect, it, vi } from "vitest";
+
+import { DirAutocomplete } from "./DirAutocomplete.js";
+
+vi.mock("../../api.js", () => ({
+  listDir: vi.fn(),
+}));
+
+import { listDir } from "../../api.js";
+
+const mockListDir = vi.mocked(listDir);
+
+afterEach(() => {
+  cleanup();
+  vi.clearAllMocks();
+});
+
+function Harness({ initial = "" }: { initial?: string }) {
+  const [value, setValue] = useState(initial);
+  return <DirAutocomplete value={value} onChange={setValue} placeholder="cwd" />;
+}
+
+// `fireEvent.focus`/`.blur` dispatch the event without moving the browser's
+// real focus, so `document.activeElement` (which the dropdown's fetch guards
+// against) never changes — calling the DOM method directly does both.
+function focus(el: HTMLElement): void {
+  act(() => el.focus());
+}
+function blur(el: HTMLElement): void {
+  act(() => el.blur());
+}
+
+describe("DirAutocomplete", () => {
+  it("lists entries matching the trailing segment on focus", async () => {
+    mockListDir.mockResolvedValue({ path: "/home/n", entries: ["alpha", "beta"] });
+    render(<Harness initial="/home/n/al" />);
+
+    focus(screen.getByPlaceholderText("cwd"));
+
+    expect(await screen.findByRole("option", { name: "alpha/" })).not.toBeNull();
+    expect(screen.queryByRole("option", { name: "beta/" })).toBeNull();
+  });
+
+  it("Tab commits the highlighted entry and does not close the field", async () => {
+    mockListDir.mockResolvedValue({ path: "/home/n", entries: ["alpha"] });
+    render(<Harness initial="/home/n/a" />);
+    const input = screen.getByPlaceholderText("cwd") as HTMLInputElement;
+    focus(input);
+    await screen.findByRole("option", { name: "alpha/" });
+
+    fireEvent.keyDown(input, { key: "Tab" });
+
+    expect(input.value).toBe("/home/n/alpha/");
+  });
+
+  it("arrows move the highlight and Enter accepts it without submitting", async () => {
+    mockListDir.mockResolvedValue({ path: "/home/n", entries: ["alpha", "beta"] });
+    render(<Harness initial="/home/n/" />);
+    const input = screen.getByPlaceholderText("cwd") as HTMLInputElement;
+    focus(input);
+    await screen.findByRole("option", { name: "alpha/" });
+
+    fireEvent.keyDown(input, { key: "ArrowDown" });
+    const notPrevented = fireEvent.keyDown(input, { key: "Enter" });
+
+    expect(input.value).toBe("/home/n/beta/");
+    expect(notPrevented).toBe(false); // preventDefault called -> event "processed"
+  });
+
+  it("Escape closes the dropdown and stops the key from bubbling", async () => {
+    mockListDir.mockResolvedValue({ path: "/home/n", entries: ["alpha"] });
+    const onKeyDown = vi.fn();
+    render(
+      <div onKeyDown={onKeyDown}>
+        <Harness initial="/home/n/a" />
+      </div>,
+    );
+    const input = screen.getByPlaceholderText("cwd");
+    focus(input);
+    await screen.findByRole("option", { name: "alpha/" });
+
+    fireEvent.keyDown(input, { key: "Escape" });
+
+    await waitFor(() => expect(screen.queryByRole("option")).toBeNull());
+    expect(onKeyDown).not.toHaveBeenCalled();
+  });
+
+  it("does not reopen on refocus until the operator types again", async () => {
+    mockListDir.mockResolvedValue({ path: "/home/n", entries: ["alpha"] });
+    render(<Harness initial="/home/n/a" />);
+    const input = screen.getByPlaceholderText("cwd");
+    focus(input);
+    await screen.findByRole("option", { name: "alpha/" });
+    fireEvent.keyDown(input, { key: "Escape" });
+    await waitFor(() => expect(screen.queryByRole("option")).toBeNull());
+
+    blur(input);
+    focus(input);
+
+    await new Promise((r) => setTimeout(r, 10));
+    expect(screen.queryByRole("option")).toBeNull();
+
+    fireEvent.change(input, { target: { value: "/home/n/al" } });
+    expect(await screen.findByRole("option", { name: "alpha/" })).not.toBeNull();
+  });
+});

web/src/components/spawn/DirAutocomplete.tsx

@@ -0,0 +1,256 @@
+// React port of ../../dirAutocomplete.ts: shell-style directory autocomplete
+// on a path <input>. Lists one directory at a time over `GET /api/dirs`,
+// caches per directory, and filters the trailing path segment locally.
+//
+// Keyboard model mirrors `cd dir/sub<tab>`:
+//   Tab    commit the highlighted entry and let focus move on; with nothing
+//          highlighted, plain tab navigation
+//   ↑/↓    move the highlight
+//   Enter  accept the highlighted entry (and suppress form submit); with
+//          nothing highlighted, bubble to the form unprevented
+//   Esc    close the dropdown, refocus the input, and stop the key from
+//          bubbling — so dismissing the popup can't also close a parent modal
+//
+// Once dismissed (Esc or blur) the popup stays closed until the operator
+// types again — refocusing alone doesn't reopen it.
+
+import { useCallback, useEffect, useRef, useState, type InputHTMLAttributes, type Ref } from "react";
+import { createPortal } from "react-dom";
+
+import { listDir } from "../../api.js";
+import { splitPath } from "../../paths.js";
+
+/** Most entries rendered at once — a long list is unhelpful and slow. */
+const MAX_VISIBLE = 60;
+/** Debounce before a network fetch when the input crosses into a new directory. */
+const FETCH_DEBOUNCE_MS = 90;
+
+export interface DirAutocompleteProps
+  extends Omit<InputHTMLAttributes<HTMLInputElement>, "value" | "onChange"> {
+  value: string;
+  onChange(value: string): void;
+  /** Access to the underlying `<input>` — e.g. to focus/select it. */
+  inputRef?: Ref<HTMLInputElement>;
+}
+
+function assignRef(ref: Ref<HTMLInputElement> | undefined, el: HTMLInputElement | null): void {
+  if (!ref) return;
+  if (typeof ref === "function") ref(el);
+  else (ref as { current: HTMLInputElement | null }).current = el;
+}
+
+export function DirAutocomplete({ value, onChange, inputRef, ...inputProps }: DirAutocompleteProps) {
+  const localRef = useRef<HTMLInputElement | null>(null);
+  const cache = useRef(new Map<string, string[]>());
+  const fetchSeq = useRef(0);
+  const debounceTimer = useRef<number | undefined>(undefined);
+  const dismissedRef = useRef(false);
+
+  const [filtered, setFiltered] = useState<string[]>([]);
+  const [highlight, setHighlight] = useState(-1);
+  const [open, setOpen] = useState(false);
+  const [rect, setRect] = useState<{ left: number; top: number; width: number } | null>(null);
+
+  const setInputRef = useCallback(
+    (el: HTMLInputElement | null) => {
+      localRef.current = el;
+      assignRef(inputRef, el);
+    },
+    [inputRef],
+  );
+
+  const reposition = useCallback(() => {
+    const el = localRef.current;
+    if (!el) return;
+    const r = el.getBoundingClientRect();
+    setRect({ left: r.left, top: r.bottom + 4, width: r.width });
+  }, []);
+
+  const close = useCallback(() => {
+    setOpen(false);
+    setFiltered([]);
+    setHighlight(-1);
+  }, []);
+
+  const dismiss = useCallback(() => {
+    dismissedRef.current = true;
+    close();
+  }, [close]);
+
+  const applyFilter = useCallback(
+    (entries: string[], prefix: string) => {
+      // A fetch can outlive the dismissal it was meant to serve (Tab-commit
+      // re-arms a refresh, then blur dismisses 120ms later). Showing it now
+      // would anchor a dropdown under an unfocused input nothing closes.
+      if (dismissedRef.current || document.activeElement !== localRef.current) {
+        close();
+        return;
+      }
+      const lower = prefix.toLowerCase();
+      const next = entries.filter((e) => e.toLowerCase().startsWith(lower));
+      setFiltered(next);
+      setHighlight(next.length > 0 ? 0 : -1);
+      if (next.length === 0) {
+        close();
+        return;
+      }
+      setOpen(true);
+      reposition();
+    },
+    [close, reposition],
+  );
+
+  const refresh = useCallback(
+    (raw: string) => {
+      const { dir, prefix } = splitPath(raw);
+      const cached = cache.current.get(dir);
+      if (cached) {
+        applyFilter(cached, prefix);
+        return;
+      }
+      const seq = ++fetchSeq.current;
+      listDir(dir)
+        .then((listing) => {
+          cache.current.set(dir, listing.entries);
+          if (seq !== fetchSeq.current) return; // a newer fetch superseded this one
+          const cur = splitPath(localRef.current?.value ?? raw);
+          if (cur.dir === dir) applyFilter(listing.entries, cur.prefix);
+        })
+        .catch(() => {
+          if (seq === fetchSeq.current) close();
+        });
+    },
+    [applyFilter, close],
+  );
+
+  const scheduleRefresh = useCallback(
+    (raw: string) => {
+      window.clearTimeout(debounceTimer.current);
+      debounceTimer.current = window.setTimeout(() => refresh(raw), FETCH_DEBOUNCE_MS);
+    },
+    [refresh],
+  );
+
+  /** Complete `name` as the next path segment and append `/` so the child
+   * directory's listing loads on the next interaction — the drill-in
+   * gesture. Unlike `accept`, doesn't steal focus: Tab uses this to commit
+   * and move to the next field in one gesture. */
+  const commit = useCallback(
+    (name: string) => {
+      const { dir } = splitPath(value);
+      onChange(`${dir}${name}/`);
+    },
+    [value, onChange],
+  );
+
+  const accept = useCallback(
+    (name: string) => {
+      commit(name);
+      localRef.current?.focus();
+    },
+    [commit],
+  );
+
+  useEffect(() => {
+    if (!open) return;
+    window.addEventListener("scroll", reposition, true);
+    window.addEventListener("resize", reposition);
+    return () => {
+      window.removeEventListener("scroll", reposition, true);
+      window.removeEventListener("resize", reposition);
+    };
+  }, [open, reposition]);
+
+  useEffect(() => () => window.clearTimeout(debounceTimer.current), []);
+
+  return (
+    <>
+      <input
+        {...inputProps}
+        ref={setInputRef}
+        value={value}
+        autoComplete="off"
+        role="combobox"
+        aria-expanded={open}
+        aria-autocomplete="list"
+        onChange={(e) => {
+          dismissedRef.current = false;
+          const next = e.currentTarget.value;
+          onChange(next);
+          scheduleRefresh(next);
+        }}
+        onFocus={(e) => {
+          inputProps.onFocus?.(e);
+          if (dismissedRef.current) return;
+          refresh(value);
+        }}
+        onBlur={(e) => {
+          inputProps.onBlur?.(e);
+          // Delay so a dropdown mousedown can complete first.
+          window.setTimeout(dismiss, 120);
+        }}
+        onKeyDown={(e) => {
+          inputProps.onKeyDown?.(e);
+          if (e.defaultPrevented) return;
+          if (e.key === "Escape") {
+            if (open) {
+              e.preventDefault();
+              e.stopPropagation();
+              dismiss();
+              localRef.current?.focus();
+            }
+            return;
+          }
+          if (e.key === "Tab") {
+            if (!open || highlight < 0 || highlight >= filtered.length) return; // plain tab navigation
+            commit(filtered[highlight]); // no preventDefault — focus moves to the next field
+            return;
+          }
+          if (!open) return;
+          if (e.key === "ArrowDown") {
+            e.preventDefault();
+            setHighlight((h) => Math.min(h + 1, filtered.length - 1));
+          } else if (e.key === "ArrowUp") {
+            e.preventDefault();
+            setHighlight((h) => Math.max(h - 1, 0));
+          } else if (e.key === "Enter" && highlight >= 0 && highlight < filtered.length) {
+            e.preventDefault(); // accept the entry instead of submitting the form
+            accept(filtered[highlight]);
+          }
+        }}
+      />
+      {open &&
+        rect &&
+        createPortal(
+          <ul
+            role="listbox"
+            className="dir-ac fixed z-modal max-h-[240px] overflow-y-auto rounded-r-2 border border-border-strong bg-surface py-sp-1 font-mono shadow-2"
+            style={{ left: rect.left, top: rect.top, width: rect.width }}
+          >
+            {filtered.slice(0, MAX_VISIBLE).map((name, i) => (
+              <li
+                key={name}
+                role="option"
+                aria-selected={i === highlight}
+                className={`cursor-pointer truncate px-sp-3 py-1 text-fs-12 text-ink ${
+                  i === highlight ? "bg-accent-tint" : ""
+                }`}
+                onMouseDown={(e) => {
+                  e.preventDefault(); // accept before the input's blur fires
+                  accept(name);
+                }}
+              >
+                {name}/
+              </li>
+            ))}
+            {filtered.length > MAX_VISIBLE && (
+              <li className="px-sp-3 py-1 text-fs-11 text-faint">
+                +{filtered.length - MAX_VISIBLE} more — keep typing to narrow
+              </li>
+            )}
+          </ul>,
+          document.body,
+        )}
+    </>
+  );
+}

web/src/components/spawn/SpawnForm.test.tsx

@@ -0,0 +1,383 @@
+// @vitest-environment happy-dom
+import { cleanup, fireEvent, render, screen, waitFor } from "@testing-library/react";
+import { afterEach, describe, expect, it, vi } from "vitest";
+
+import type { BranchInfo, DaemonInfo, Project, Session, SpawnPreset } from "../../types.js";
+import { SpawnForm } from "./SpawnForm.js";
+
+vi.mock("../../api.js", () => ({
+  createSession: vi.fn(),
+  fetchBranches: vi.fn(),
+  fetchNames: vi.fn(),
+  getDaemonInfo: vi.fn(),
+  gitRoot: vi.fn(),
+  listDir: vi.fn(),
+  projects: vi.fn(),
+}));
+
+import {
+  createSession,
+  fetchBranches,
+  fetchNames,
+  getDaemonInfo,
+  gitRoot,
+  listDir,
+  projects as fetchProjects,
+} from "../../api.js";
+
+const mockCreateSession = vi.mocked(createSession);
+const mockFetchBranches = vi.mocked(fetchBranches);
+const mockFetchNames = vi.mocked(fetchNames);
+const mockGetDaemonInfo = vi.mocked(getDaemonInfo);
+const mockGitRoot = vi.mocked(gitRoot);
+const mockListDir = vi.mocked(listDir);
+const mockFetchProjects = vi.mocked(fetchProjects);
+
+const presets: SpawnPreset[] = [
+  { id: "claude", label: "Claude", command: "claude" },
+  { id: "opencode", label: "opencode", command: "opencode" },
+  { id: "zsh", label: "Shell", command: "zsh" },
+  { id: "custom", label: "Custom…", command: "" },
+];
+
+function info(over: Partial<DaemonInfo> = {}): DaemonInfo {
+  return { hostname: "h", version: "1", home_dir: "/home/n", presets, ...over };
+}
+
+function project(over: Partial<Project> = {}): Project {
+  return { path: "/repos/p", name: "p", last_activity: new Date().toISOString(), ...over };
+}
+
+function session(over: Partial<Session> = {}): Session {
+  return {
+    id: "s1",
+    cwd: "/repos/p",
+    command: "claude",
+    env: {},
+    metadata: {},
+    status: "Running",
+    created_at: "2026-01-01T00:00:00Z",
+    ...over,
+  };
+}
+
+/** Defaults every test starts from: one known project, no branches, a fresh
+ * generated handle. Individual tests override what they care about. */
+function stub(over: { projects?: Project[]; info?: DaemonInfo; names?: string[]; branches?: BranchInfo[] } = {}) {
+  mockGetDaemonInfo.mockResolvedValue(over.info ?? info());
+  mockFetchProjects.mockResolvedValue(over.projects ?? [project()]);
+  mockFetchNames.mockResolvedValue({ names: over.names ?? ["amber-fox"] });
+  mockFetchBranches.mockResolvedValue(over.branches ?? []);
+  mockGitRoot.mockResolvedValue({});
+  mockListDir.mockResolvedValue({ path: "", entries: [] });
+}
+
+afterEach(() => {
+  cleanup();
+  vi.clearAllMocks();
+});
+
+async function settle() {
+  await waitFor(() => expect(screen.queryByText("Loading projects…")).toBeNull());
+}
+
+/** Everything worktree/name/structured-related lives behind the "Advanced"
+ * disclosure, collapsed by default (#93) — mirrors the vanilla form exactly. */
+function openAdvanced() {
+  fireEvent.click(screen.getByRole("button", { name: /Advanced/ }));
+}
+
+describe("SpawnForm harness picker", () => {
+  it("selects the first non-custom preset by default and reveals the custom input on Custom…", async () => {
+    stub();
+    render(<SpawnForm onSuccess={vi.fn()} />);
+    await settle();
+
+    expect(screen.getByRole("button", { name: /^Claude/ }).className).toContain("border-accent");
+    fireEvent.click(screen.getByRole("button", { name: "Custom…" }));
+
+    expect(screen.getByPlaceholderText("bash -l").hidden).toBe(false);
+  });
+
+  it("resolves a seeded command to its matching preset", async () => {
+    stub();
+    render(<SpawnForm seed={{ command: "opencode" }} onSuccess={vi.fn()} />);
+    await settle();
+
+    const opencode = screen.getByRole("button", { name: /opencode/ });
+    expect(opencode.className).toContain("border-accent");
+  });
+
+  it("drops an unmatched seeded command into the custom field", async () => {
+    stub();
+    render(<SpawnForm seed={{ command: "hx src/" }} onSuccess={vi.fn()} />);
+    await settle();
+
+    const raw = screen.getByPlaceholderText("bash -l") as HTMLInputElement;
+    expect(raw.hidden).toBe(false);
+    expect(raw.value).toBe("hx src/");
+  });
+});
+
+describe("SpawnForm project picker", () => {
+  it("lists discovered projects and reveals the worktree step on pick", async () => {
+    stub({ projects: [project({ path: "/repos/a", name: "a" }), project({ path: "/repos/b", name: "b" })] });
+    render(<SpawnForm onSuccess={vi.fn()} />);
+    await settle();
+    openAdvanced();
+
+    expect(screen.queryByText("Worktree")).toBeNull();
+    fireEvent.click(screen.getByText("a"));
+
+    expect(screen.getByText("Worktree")).not.toBeNull();
+    expect(screen.getByText("+ new").closest("button")?.getAttribute("data-active")).toBe("true");
+  });
+
+  it("filters the project grid by name", async () => {
+    stub({ projects: [project({ path: "/repos/alpha", name: "alpha" }), project({ path: "/repos/beta", name: "beta" })] });
+    render(<SpawnForm onSuccess={vi.fn()} />);
+    await settle();
+
+    fireEvent.change(screen.getByPlaceholderText("Filter projects…"), { target: { value: "alp" } });
+
+    expect(screen.getByText("alpha")).not.toBeNull();
+    expect(screen.queryByText("beta")).toBeNull();
+  });
+
+  it("falls back to the raw directory input when no projects are discovered", async () => {
+    stub({ projects: [] });
+    render(<SpawnForm onSuccess={vi.fn()} />);
+    await settle();
+
+    expect((screen.getByPlaceholderText("/home/user/project") as HTMLInputElement).value).toBe("/home/n");
+  });
+
+  it("Other directory… clears the picker and opens the raw input", async () => {
+    stub({ projects: [project({ path: "/repos/a", name: "a" })] });
+    render(<SpawnForm onSuccess={vi.fn()} />);
+    await settle();
+    fireEvent.click(screen.getByText("a"));
+    expect(screen.getByText("a").closest("button")?.getAttribute("data-active")).toBe("true");
+
+    fireEvent.click(screen.getByText("Other directory…"));
+
+    expect(screen.getByText("a").closest("button")?.getAttribute("data-active")).toBeNull();
+    expect((screen.getByPlaceholderText("/home/user/project") as HTMLInputElement).value).toBe("/repos/a");
+  });
+});
+
+describe("SpawnForm worktree defaults", () => {
+  it("defaults the toggle on for a picked project and infers the repo without a gitroot probe", async () => {
+    stub({ projects: [project({ path: "/repos/a", name: "a" })] });
+    render(<SpawnForm onSuccess={vi.fn()} />);
+    await settle();
+    openAdvanced();
+
+    fireEvent.click(screen.getByText("a"));
+
+    expect(screen.getByText("repo: /repos/a")).not.toBeNull();
+    expect(mockGitRoot).not.toHaveBeenCalled();
+  });
+
+  it("defaults the toggle off for a project that is itself a worktree (#91)", async () => {
+    stub({
+      projects: [
+        project({ path: "/repos/main", name: "main" }),
+        project({ path: "/repos/wt", name: "wt", is_worktree: true, base_repo: "/repos/main" }),
+      ],
+    });
+    render(<SpawnForm onSuccess={vi.fn()} />);
+    await settle();
+    openAdvanced();
+
+    fireEvent.click(screen.getByText("main"));
+    fireEvent.click(screen.getByText("wt"));
+
+    expect((screen.getByLabelText("Create git worktree") as HTMLInputElement).checked).toBe(false);
+  });
+
+  it("still probes gitroot for a free-text directory", async () => {
+    stub();
+    mockGitRoot.mockResolvedValue({ root: "/repos/found" });
+    render(<SpawnForm onSuccess={vi.fn()} />);
+    await settle();
+    openAdvanced();
+    fireEvent.click(screen.getByText("Other directory…"));
+
+    fireEvent.change(screen.getByPlaceholderText("/home/user/project"), {
+      target: { value: "/repos/found/sub" },
+    });
+
+    await waitFor(() => expect(mockGitRoot).toHaveBeenCalledWith("/repos/found/sub"));
+    expect(await screen.findByText("repo: /repos/found")).not.toBeNull();
+  });
+
+  it("populates existing branches, disabling the one held by another worktree", async () => {
+    stub({
+      projects: [project({ path: "/repos/a", name: "a" })],
+      branches: [
+        { name: "main", is_head: true },
+        { name: "feat/x", is_head: false, worktree: "/repos/other" },
+        { name: "feat/y", is_head: false },
+      ],
+    });
+    render(<SpawnForm onSuccess={vi.fn()} />);
+    await settle();
+    openAdvanced();
+    fireEvent.click(screen.getByText("a"));
+
+    fireEvent.click(screen.getByLabelText("Existing branch"));
+
+    const y = await screen.findByRole("option", { name: "feat/y" });
+    expect((y as HTMLOptionElement).disabled).toBe(false);
+    expect((screen.getByRole("option", { name: /main — held by the main checkout/ }) as HTMLOptionElement).disabled).toBe(
+      true,
+    );
+  });
+
+  it("submits the first pickable branch when the operator never opens the select", async () => {
+    stub({
+      projects: [project({ path: "/repos/a", name: "a" })],
+      branches: [
+        { name: "main", is_head: true },
+        { name: "feat/y", is_head: false },
+      ],
+    });
+    mockCreateSession.mockResolvedValue(session());
+    const onSuccess = vi.fn();
+    render(<SpawnForm onSuccess={onSuccess} />);
+    await settle();
+    openAdvanced();
+    fireEvent.click(screen.getByText("a"));
+    fireEvent.click(screen.getByLabelText("Existing branch"));
+    await screen.findByRole("option", { name: "feat/y" });
+
+    fireEvent.click(screen.getByRole("button", { name: "Spawn" }));
+
+    await waitFor(() => expect(onSuccess).toHaveBeenCalled());
+    expect(mockCreateSession).toHaveBeenCalledWith(
+      expect.objectContaining({ worktree: expect.objectContaining({ branch: "feat/y", create_branch: false }) }),
+    );
+  });
+});
+
+describe("SpawnForm structured view", () => {
+  it("defaults on for a resolved Claude Code command and off for an unsupported one", async () => {
+    stub();
+    render(<SpawnForm onSuccess={vi.fn()} />);
+    await settle();
+    openAdvanced();
+
+    expect((screen.getByLabelText("Structured view") as HTMLInputElement).checked).toBe(true);
+
+    fireEvent.click(screen.getByRole("button", { name: /^Shell/ }));
+
+    expect(screen.queryByLabelText("Structured view")).toBeNull();
+  });
+
+  it("keeps an unchecked box unchecked across a command change between two supported tools", async () => {
+    stub();
+    render(<SpawnForm onSuccess={vi.fn()} />);
+    await settle();
+    openAdvanced();
+    fireEvent.click(screen.getByLabelText("Structured view"));
+
+    fireEvent.click(screen.getByRole("button", { name: /opencode/ }));
+    fireEvent.click(screen.getByRole("button", { name: /^Claude/ }));
+
+    expect((screen.getByLabelText("Structured view") as HTMLInputElement).checked).toBe(false);
+  });
+});
+
+describe("SpawnForm submit", () => {
+  it("builds and posts the request, reporting the created session", async () => {
+    stub({ projects: [project({ path: "/repos/a", name: "a" })] });
+    const created = session();
+    mockCreateSession.mockResolvedValue(created);
+    const onSuccess = vi.fn();
+    render(<SpawnForm onSuccess={onSuccess} />);
+    await settle();
+    fireEvent.click(screen.getByText("a"));
+
+    fireEvent.click(screen.getByRole("button", { name: "Spawn" }));
+
+    await waitFor(() => expect(onSuccess).toHaveBeenCalledWith(created));
+    expect(mockCreateSession).toHaveBeenCalledWith(
+      expect.objectContaining({ cwd: "/repos/a", command: "claude" }),
+    );
+  });
+
+  it("shows the daemon's rejection and leaves the form open", async () => {
+    stub({ projects: [project({ path: "/repos/a", name: "a" })] });
+    mockCreateSession.mockRejectedValue(new Error("cwd is not a directory"));
+    render(<SpawnForm onSuccess={vi.fn()} />);
+    await settle();
+    fireEvent.click(screen.getByText("a"));
+
+    fireEvent.click(screen.getByRole("button", { name: "Spawn" }));
+
+    expect(await screen.findByText("cwd is not a directory")).not.toBeNull();
+  });
+
+  it("rejects an existing-branch worktree with nothing picked", async () => {
+    stub({ projects: [project({ path: "/repos/a", name: "a" })], branches: [] });
+    render(<SpawnForm onSuccess={vi.fn()} />);
+    await settle();
+    openAdvanced();
+    fireEvent.click(screen.getByText("a"));
+    fireEvent.click(screen.getByLabelText("Existing branch"));
+    await screen.findByText("No local branches found.");
+
+    fireEvent.click(screen.getByRole("button", { name: "Spawn" }));
+
+    expect(await screen.findByText("Pick an existing branch.")).not.toBeNull();
+    expect(mockCreateSession).not.toHaveBeenCalled();
+  });
+});
+
+describe("SpawnForm Enter-to-submit (#97)", () => {
+  it("submits from the session name field", async () => {
+    stub({ projects: [project({ path: "/repos/a", name: "a" })] });
+    mockCreateSession.mockResolvedValue(session());
+    const onSuccess = vi.fn();
+    render(<SpawnForm onSuccess={onSuccess} />);
+    await settle();
+    fireEvent.click(screen.getByText("a"));
+    openAdvanced();
+
+    fireEvent.keyDown(screen.getByPlaceholderText("named after your first prompt"), { key: "Enter" });
+
+    await waitFor(() => expect(onSuccess).toHaveBeenCalled());
+  });
+
+  it("first Enter picks a not-yet-active project box, the second submits", async () => {
+    stub({ projects: [project({ path: "/repos/a", name: "a" }), project({ path: "/repos/b", name: "b" })] });
+    mockCreateSession.mockResolvedValue(session());
+    const onSuccess = vi.fn();
+    render(<SpawnForm onSuccess={onSuccess} />);
+    await settle();
+
+    const boxB = screen.getByText("b").closest("button")!;
+    fireEvent.keyDown(boxB, { key: "Enter" });
+    expect(onSuccess).not.toHaveBeenCalled();
+    expect(boxB.getAttribute("data-active")).toBe("true");
+
+    fireEvent.keyDown(boxB, { key: "Enter" });
+    await waitFor(() => expect(onSuccess).toHaveBeenCalled());
+  });
+
+  it("toggles the worktree checkbox on Enter instead of submitting", async () => {
+    stub({ projects: [project({ path: "/repos/a", name: "a" })] });
+    render(<SpawnForm onSuccess={vi.fn()} />);
+    await settle();
+    openAdvanced();
+    fireEvent.click(screen.getByText("a"));
+    const checkbox = screen.getByLabelText("Create git worktree") as HTMLInputElement;
+    expect(checkbox.checked).toBe(true);
+
+    fireEvent.keyDown(checkbox, { key: "Enter" });
+
+    expect(checkbox.checked).toBe(false);
+    expect(mockCreateSession).not.toHaveBeenCalled();
+  });
+});

web/src/components/spawn/SpawnForm.tsx

@@ -0,0 +1,853 @@
+// React port of ../../spawnForm.ts: the spawn form itself — preset tiles,
+// two-step project picker, name, structured-view checkbox, worktree. Mounted
+// inside SpawnModal.
+//
+// Working-directory picker (#95), unchanged from the vanilla form:
+//  1. Step one is one box per repository (name, then its parent directory).
+//     A worktree is a box only when its main checkout is missing from the
+//     scan.
+//  2. The filter keeps a repository when the query matches it or any of its
+//     worktrees.
+//  3. Picking a box reveals step two: "+ new", then that repository's
+//     worktrees newest-first. "Other directory…" clears both steps.
+//  4. "+ new" is the default selection — except on a box that is itself a
+//     linked worktree, which defaults to that worktree.
+//  5. "+ new" means create: cwd is the repository, create-worktree on. An
+//     existing worktree means run in it: cwd is that worktree, create-worktree
+//     off (the daemon never adopts, docs/WORKTREES.md).
+//  6. Neither step probes /api/gitroot; only the free-text hatch does.
+
+import { useCallback, useEffect, useMemo, useRef, useState } from "react";
+
+import {
+  createSession,
+  fetchBranches,
+  fetchNames,
+  getDaemonInfo,
+  gitRoot,
+  projects as fetchProjects,
+} from "../../api.js";
+import { parentDir } from "../../paths.js";
+import {
+  filterProjects,
+  groupProjects,
+  isStale,
+  sortProjects,
+  type ProjectGroup,
+} from "../../projectList.js";
+import {
+  buildSpawnRequest,
+  FALLBACK_PRESETS,
+  supportsStructuredView,
+  type SpawnFormSeed,
+  type SpawnFormValues,
+  type WorktreeBranchMode,
+} from "../../spawnForm.js";
+import { splitCommand, toolIconSvg, TOOL_LABELS } from "../../toolIcon.js";
+import { abbreviateCwd } from "../../types.js";
+import type { BranchInfo, Project, Session, SpawnPreset } from "../../types.js";
+import { BranchPicker } from "./BranchPicker.js";
+import { DirAutocomplete } from "./DirAutocomplete.js";
+
+/** Quiet period after raw-path typing before `/api/gitroot` fires (#74). Must
+ * stay equal to `GITROOT_PROBE_DEBOUNCE_MS` in crates/maestro-tui/src/draft.rs
+ * and to ../../spawnForm.ts's own constant, so every client debounces alike. */
+const GITROOT_PROBE_DEBOUNCE_MS = 250;
+
+/** "new" (create a worktree) or the path of an existing worktree to run in. */
+type WorktreeChoice = "new" | string;
+
+type BaseRepoResolution =
+  | { kind: "disabled" }
+  | { kind: "no-cwd" }
+  | { kind: "found"; repo: string }
+  | { kind: "probe" };
+
+function resolveBaseRepoFor(cwd: string, wtEnabled: boolean, discovered: Project[]): BaseRepoResolution {
+  if (!wtEnabled) return { kind: "disabled" };
+  if (!cwd) return { kind: "no-cwd" };
+  const project = discovered.find((p) => p.path === cwd);
+  if (project) return { kind: "found", repo: project.base_repo ?? project.path };
+  return { kind: "probe" };
+}
+
+/** Every worktree of a group, including the group's own box when the scan
+ * found no main checkout to nest it under. */
+function worktreesOf(group: ProjectGroup): Project[] {
+  return [group.main, ...group.worktrees].filter((p) => p.is_worktree);
+}
+
+/** Resolve a preset id to a tool icon id — the preset's command is the source
+ * of truth, the same way the sidebar derives one. The "Custom…" preset (empty
+ * command) has no tool to show. */
+function presetToolId(p: SpawnPreset): string | null {
+  if (!p.command) return null;
+  return splitCommand(p.command).tool;
+}
+
+function harnessIcon(toolId: string): string | null {
+  const small = toolIconSvg(toolId);
+  if (!small) return null;
+  // toolIconSvg renders at 13x13 (viewBox 0 0 16 16) for the sidebar; the
+  // harness card wants the same mark at 24x24 so it breathes.
+  return small.replace(/width="13"/g, 'width="24"').replace(/height="13"/g, 'height="24"');
+}
+
+function pickerButtonClass(active: boolean): string {
+  return `flex min-h-[32px] items-baseline justify-between gap-sp-3 rounded-r-2 border px-sp-3 py-1.5 text-left font-mono ${
+    active ? "border-accent bg-accent-tint" : "border-border bg-surface-2 hover:border-border-strong"
+  }`;
+}
+
+export interface SpawnFormProps {
+  seed?: SpawnFormSeed;
+  /** Fired whenever any value changes, once the form's initial seed settles. */
+  onChange?(values: SpawnFormValues): void;
+  onSuccess(session: Session): void;
+}
+
+export function SpawnForm({ seed, onChange, onSuccess }: SpawnFormProps) {
+  const prefill = useRef(seed ?? {}).current;
+
+  const formRef = useRef<HTMLFormElement>(null);
+  const filterInputRef = useRef<HTMLInputElement>(null);
+  const customCommandRef = useRef<HTMLInputElement>(null);
+  const rawCwdRef = useRef<HTMLInputElement>(null);
+  const destroyedRef = useRef(false);
+  useEffect(() => () => {
+    destroyedRef.current = true;
+  }, []);
+
+  // ---- harness / command -------------------------------------------------
+  const [homeDir, setHomeDir] = useState("");
+  const [infoLoading, setInfoLoading] = useState(true);
+  const [presets, setPresets] = useState<SpawnPreset[]>(FALLBACK_PRESETS);
+  const [selectedPresetId, setSelectedPresetId] = useState("");
+  const [command, setCommand] = useState("");
+  const [customCommand, setCustomCommand] = useState("");
+  const [commandTouched, setCommandTouched] = useState(false);
+  const [structuredView, setStructuredView] = useState(false);
+  const [structuredTouched, setStructuredTouched] = useState(false);
+
+  const activePreset = useMemo(() => presets.find((p) => p.id === selectedPresetId), [presets, selectedPresetId]);
+  const isCustomSelected = !activePreset || activePreset.command === "";
+
+  useEffect(() => {
+    if (commandTouched && isCustomSelected) customCommandRef.current?.focus();
+  }, [isCustomSelected, commandTouched]);
+
+  useEffect(() => {
+    let cancelled = false;
+    getDaemonInfo()
+      .then((info) => {
+        if (cancelled) return;
+        setHomeDir(info.home_dir ?? "");
+        if (info.presets?.length) setPresets(info.presets);
+      })
+      .catch(() => {
+        // keep fallbacks
+      })
+      .finally(() => {
+        if (!cancelled) setInfoLoading(false);
+      });
+    return () => {
+      cancelled = true;
+    };
+  }, []);
+
+  // Pick a sensible starting command whenever the preset list changes and the
+  // operator hasn't touched it yet: a seed's command (matched to a preset, or
+  // dropped into custom), else the first non-custom preset.
+  useEffect(() => {
+    if (commandTouched) return;
+    const wanted = prefill.command;
+    if (wanted) {
+      const match = presets.find((p) => p.command === wanted && p.command !== "");
+      if (match) {
+        setSelectedPresetId(match.id);
+        setCommand(match.command);
+        return;
+      }
+      const custom = presets.find((p) => p.command === "");
+      setCustomCommand(wanted);
+      setSelectedPresetId(custom ? custom.id : "custom");
+      setCommand(wanted);
+      return;
+    }
+    const first = presets.find((p) => p.command !== "");
+    if (first) {
+      setSelectedPresetId(first.id);
+      setCommand(first.command);
+    }
+    // prefill is captured once at construction, not a reactive dependency.
+    // eslint-disable-next-line react-hooks/exhaustive-deps
+  }, [presets, commandTouched]);
+
+  const structuredSupported = useMemo(() => supportsStructuredView(command), [command]);
+  useEffect(() => {
+    if (!structuredSupported) setStructuredView(false);
+    else if (!structuredTouched) setStructuredView(true);
+  }, [structuredSupported, structuredTouched]);
+
+  // ---- project picker ------------------------------------------------------
+  const [discovered, setDiscovered] = useState<Project[]>([]);
+  const [projectsLoading, setProjectsLoading] = useState(true);
+  const [filterQuery, setFilterQuery] = useState("");
+  const [cwd, setCwd] = useState("");
+  const [cwdRawVisible, setCwdRawVisible] = useState(false);
+  const [rawFocusToken, setRawFocusToken] = useState(0);
+  const [selectedGroupPath, setSelectedGroupPath] = useState<string | null>(null);
+  const [worktreeChoice, setWorktreeChoice] = useState<WorktreeChoice>("new");
+
+  const groups = useMemo(() => groupProjects(sortProjects(discovered, new Date())), [discovered]);
+  const visibleGroups = useMemo(
+    () => groups.filter((g) => filterProjects([g.main, ...g.worktrees], filterQuery).length > 0),
+    [groups, filterQuery],
+  );
+  const selectedGroup = useMemo(
+    () => groups.find((g) => g.main.path === selectedGroupPath) ?? null,
+    [groups, selectedGroupPath],
+  );
+
+
+  useEffect(() => {
+    let cancelled = false;
+    fetchProjects()
+      .then((list) => {
+        if (!cancelled) setDiscovered(list);
+      })
+      .catch(() => {
+        if (!cancelled) setDiscovered([]);
+      })
+      .finally(() => {
+        if (!cancelled) setProjectsLoading(false);
+      });
+    return () => {
+      cancelled = true;
+    };
+  }, []);
+
+  // ---- worktree (branch-only; base_repo inferred via scan or /gitroot) ----
+  const [wtEnabled, setWtEnabled] = useState(prefill.worktreeEnabled ?? true);
+  const [inferredBaseRepo, setInferredBaseRepo] = useState<string | null>(null);
+  const [wtHint, setWtHint] = useState<string | null>(null);
+  const [name, setName] = useState(prefill.name ?? "");
+  const [branchEdited, setBranchEdited] = useState(Boolean(prefill.worktreeBranchEdited));
+  const [branchMode, setBranchMode] = useState<WorktreeBranchMode>(prefill.worktreeBranchMode ?? "new");
+  const [newBranch, setNewBranch] = useState(prefill.worktreeBranch ?? "");
+  const [existingBranch, setExistingBranch] = useState("");
+  const [branchCache, setBranchCache] = useState<{ cwd: string; branches: BranchInfo[] } | null>(null);
+
+  const [advancedCollapsed, setAdvancedCollapsed] = useState(true);
+  const [error, setError] = useState("");
+  const [submitting, setSubmitting] = useState(false);
+  const [ready, setReady] = useState(false);
+
+  const probeTokenRef = useRef(0);
+  const probeTimerRef = useRef<number | undefined>(undefined);
+
+  const applyBaseRepoResolution = useCallback((res: BaseRepoResolution) => {
+    switch (res.kind) {
+      case "disabled":
+        setInferredBaseRepo(null);
+        setWtHint(null);
+        return;
+      case "no-cwd":
+        setInferredBaseRepo(null);
+        setWtHint("Choose a working directory first.");
+        return;
+      case "found":
+        setInferredBaseRepo(res.repo);
+        setWtHint(null);
+        setBranchCache(null);
+        setExistingBranch("");
+        return;
+      case "probe":
+        return; // caller schedules the probe
+    }
+  }, []);
+
+  const probeGitRoot = useCallback(async (path: string) => {
+    const token = ++probeTokenRef.current;
+    if (!path) {
+      setInferredBaseRepo(null);
+      setWtHint("Choose a working directory first.");
+      return;
+    }
+    setWtHint("Checking for a git repository…");
+    let root: string | undefined;
+    try {
+      root = (await gitRoot(path)).root;
+    } catch {
+      root = undefined;
+    }
+    if (token !== probeTokenRef.current || destroyedRef.current) return; // a newer probe superseded us
+    if (root) {
+      setInferredBaseRepo(root);
+      setWtHint(null);
+      setBranchCache(null);
+      setExistingBranch("");
+    } else {
+      setInferredBaseRepo(null);
+      // Publish the untick too, or a later submit fails with an inferred repo
+      // that no longer matches an unticked box.
+      setWtEnabled(false);
+      setWtHint("Not inside a git repository — pick a dir in a repo to use a worktree.");
+    }
+  }, []);
+
+  const scheduleGitRootProbe = useCallback(
+    (path: string) => {
+      window.clearTimeout(probeTimerRef.current);
+      probeTimerRef.current = window.setTimeout(() => {
+        probeTimerRef.current = undefined;
+        void probeGitRoot(path);
+      }, GITROOT_PROBE_DEBOUNCE_MS);
+    },
+    [probeGitRoot],
+  );
+  useEffect(() => () => window.clearTimeout(probeTimerRef.current), []);
+
+  // Seed the form once the initial project scan and daemon info have both
+  // settled — mirrors createSpawnForm's `loadProjects()`. Computed from local
+  // values rather than reading state back, since the setState calls below
+  // won't have committed yet within this same effect.
+  const seededRef = useRef(false);
+  useEffect(() => {
+    if (infoLoading || projectsLoading || seededRef.current) return;
+    seededRef.current = true;
+
+    const seedCwd = prefill.cwd ?? null;
+    let finalCwd = "";
+    let finalCwdRawVisible = false;
+    let finalGroupPath: string | null = null;
+    let finalChoice: WorktreeChoice = "new";
+
+    const restore = (path: string): boolean => {
+      for (const group of groups) {
+        const wt = worktreesOf(group).find((p) => p.path === path);
+        const choice: WorktreeChoice | null = wt ? wt.path : group.main.path === path ? "new" : null;
+        if (choice === null) continue;
+        finalGroupPath = group.main.path;
+        finalChoice = choice;
+        return true;
+      }
+      return false;
+    };
+
+    if (seedCwd) {
+      if (prefill.cwdRawVisible === false) {
+        finalCwd = seedCwd;
+        if (!restore(seedCwd)) finalCwdRawVisible = true;
+      } else {
+        finalCwd = seedCwd;
+        finalCwdRawVisible = true;
+      }
+    } else if (discovered.length === 0) {
+      finalCwd = homeDir;
+      finalCwdRawVisible = true;
+    }
+
+    setCwd(finalCwd);
+    setCwdRawVisible(finalCwdRawVisible);
+    setSelectedGroupPath(finalGroupPath);
+    setWorktreeChoice(finalChoice);
+
+    const res = resolveBaseRepoFor(finalCwd, wtEnabled, discovered);
+    applyBaseRepoResolution(res);
+    if (res.kind === "probe") scheduleGitRootProbe(finalCwd);
+
+    setReady(true);
+    // groups/wtEnabled/discovered/homeDir are read for this one settle pass;
+    // re-running on their later changes would re-seed a form the operator
+    // has since edited.
+    // eslint-disable-next-line react-hooks/exhaustive-deps
+  }, [infoLoading, projectsLoading]);
+
+  // ---- name (also seeds the worktree branch) ------------------------------
+  const skipInitialBranchSync = useRef(Boolean(prefill.worktreeBranch));
+  useEffect(() => {
+    // One-time construction seeding, mirroring spawnForm.ts precedence: an
+    // explicit seed wins outright; otherwise the prefilled name; otherwise a
+    // freshly generated handle (#53).
+    if (prefill.worktreeBranch) return;
+    if (!branchEdited && branchMode === "new" && prefill.name) return; // handled below, mount-only
+    if (!branchEdited && branchMode === "new") {
+      void fetchNames(1)
+        .then((batch) => {
+          const handle = batch.names[0];
+          if (destroyedRef.current || !handle) return;
+          setNewBranch((cur) => cur || handle);
+        })
+        .catch(() => {
+          // the operator types their own branch
+        });
+    }
+    // eslint-disable-next-line react-hooks/exhaustive-deps
+  }, []);
+
+  useEffect(() => {
+    if (!skipInitialBranchSync.current && !branchEdited && branchMode === "new" && name) {
+      setNewBranch(name);
+    }
+    skipInitialBranchSync.current = false;
+    // A cleared name leaves the branch on its last value; only `name` changes
+    // (typed or the mount-time prefill) drive this sync.
+    // eslint-disable-next-line react-hooks/exhaustive-deps
+  }, [name]);
+
+  // ---- existing-branch picker (#61): fetch once per cwd -------------------
+  useEffect(() => {
+    if (branchMode !== "existing" || !cwd || !inferredBaseRepo) return;
+    if (branchCache?.cwd === cwd) return;
+    let cancelled = false;
+    const target = cwd;
+    fetchBranches(target)
+      .then((branches) => {
+        if (cancelled || destroyedRef.current) return;
+        setBranchCache({ cwd: target, branches });
+      })
+      .catch(() => {
+        // the picker just stays empty; "new" mode still works
+      });
+    return () => {
+      cancelled = true;
+    };
+  }, [branchMode, cwd, inferredBaseRepo, branchCache]);
+
+  const branchesForCwd = branchCache?.cwd === cwd ? branchCache.branches : undefined;
+
+  // A native <select> defaults its displayed (and reported) value to the
+  // first enabled option once nothing is explicitly selected. Derived at
+  // render/submit time rather than synced via an effect, so there is no
+  // window where the control shows one branch while state still holds "".
+  const effectiveExistingBranch =
+    existingBranch || branchesForCwd?.find((b) => !b.worktree && !b.is_head)?.name || "";
+
+  // ---- emit (#observability parity with spawnForm.ts's onChange) ---------
+  useEffect(() => {
+    if (!ready) return;
+    onChange?.({
+      command,
+      cwd,
+      name,
+      structuredView,
+      autoApprove: activePreset?.auto_approve ?? false,
+      worktreeEnabled: wtEnabled,
+      worktreeBranch: branchMode === "existing" ? effectiveExistingBranch : newBranch,
+      worktreeBranchEdited: branchEdited,
+      worktreeBranchMode: branchMode,
+      cwdRawVisible,
+    });
+  }, [
+    ready,
+    command,
+    cwd,
+    name,
+    structuredView,
+    activePreset,
+    wtEnabled,
+    branchMode,
+    effectiveExistingBranch,
+    newBranch,
+    branchEdited,
+    cwdRawVisible,
+    onChange,
+  ]);
+
+  // ---- handlers -------------------------------------------------------------
+  const selectPreset = useCallback(
+    (id: string) => {
+      setCommandTouched(true);
+      setSelectedPresetId(id);
+      const preset = presets.find((p) => p.id === id);
+      if (!preset || preset.command === "") setCommand(customCommand.trim());
+      else setCommand(preset.command);
+    },
+    [presets, customCommand],
+  );
+
+  const clearProjectSelection = useCallback(() => {
+    setSelectedGroupPath(null);
+    setWorktreeChoice("new");
+  }, []);
+
+  const showRawCwd = useCallback(
+    (prefillValue?: string, focus = true) => {
+      setCwdRawVisible(true);
+      if (prefillValue != null) setCwd(prefillValue);
+      clearProjectSelection();
+      if (focus) setRawFocusToken((t) => t + 1);
+    },
+    [clearProjectSelection],
+  );
+
+  useEffect(() => {
+    if (rawFocusToken === 0) return;
+    rawCwdRef.current?.focus();
+    rawCwdRef.current?.select();
+  }, [rawFocusToken]);
+
+  const applyWorktreeChoice = useCallback(
+    (group: ProjectGroup, choice: WorktreeChoice) => {
+      const target = choice === "new" ? group.main : worktreesOf(group).find((p) => p.path === choice);
+      if (!target) return;
+      setWorktreeChoice(choice);
+      setCwd(target.path);
+      setBranchCache(null);
+      setExistingBranch("");
+      if (choice === "new") {
+        setWtEnabled(true);
+        applyBaseRepoResolution({ kind: "found", repo: group.main.base_repo ?? group.main.path });
+      } else {
+        setWtEnabled(false);
+        applyBaseRepoResolution({ kind: "disabled" });
+      }
+    },
+    [applyBaseRepoResolution],
+  );
+
+  const selectProject = useCallback(
+    (group: ProjectGroup) => {
+      setSelectedGroupPath(group.main.path);
+      setCwdRawVisible(false);
+      // Rule 4: a box that is itself a linked worktree defaults to running there.
+      applyWorktreeChoice(group, group.main.is_worktree ? group.main.path : "new");
+    },
+    [applyWorktreeChoice],
+  );
+
+  const onWtEnabledChange = useCallback(
+    (next: boolean) => {
+      setWtEnabled(next);
+      const res = resolveBaseRepoFor(cwd, next, discovered);
+      applyBaseRepoResolution(res);
+      if (res.kind === "probe") scheduleGitRootProbe(cwd);
+    },
+    [cwd, discovered, applyBaseRepoResolution, scheduleGitRootProbe],
+  );
+
+  const onRawCwdChange = useCallback(
+    (v: string) => {
+      setCwd(v);
+      setBranchCache(null);
+      setExistingBranch("");
+      if (wtEnabled) scheduleGitRootProbe(v);
+    },
+    [wtEnabled, scheduleGitRootProbe],
+  );
+
+  const onSubmit = useCallback(
+    async (e: React.FormEvent<HTMLFormElement>) => {
+      e.preventDefault();
+      setError("");
+      const values: SpawnFormValues = {
+        command,
+        cwd,
+        name,
+        structuredView,
+        autoApprove: activePreset?.auto_approve ?? false,
+        worktreeEnabled: wtEnabled,
+        worktreeBranch: branchMode === "existing" ? effectiveExistingBranch : newBranch,
+        worktreeBranchEdited: branchEdited,
+        worktreeBranchMode: branchMode,
+      };
+      const built = buildSpawnRequest(values, inferredBaseRepo);
+      if ("error" in built) {
+        setError(built.error);
+        return;
+      }
+      setSubmitting(true);
+      try {
+        const session = await createSession(built.req);
+        onSuccess(session);
+      } catch (err) {
+        setError((err as Error).message);
+        setSubmitting(false);
+      }
+    },
+    [
+      command,
+      cwd,
+      name,
+      structuredView,
+      activePreset,
+      wtEnabled,
+      branchMode,
+      effectiveExistingBranch,
+      newBranch,
+      branchEdited,
+      inferredBaseRepo,
+      onSuccess,
+    ],
+  );
+
+  // ---- Enter submits from anywhere (#97) ------------------------------------
+  const onFormKeyDown = useCallback(
+    (e: React.KeyboardEvent<HTMLFormElement>) => {
+      if (e.key !== "Enter" || e.defaultPrevented) return;
+      const target = e.target as HTMLElement;
+
+      if (target.dataset.role === "toggle") {
+        e.preventDefault();
+        (target as HTMLInputElement).click();
+        return;
+      }
+
+      if (target instanceof HTMLButtonElement && target.dataset.picker) {
+        e.preventDefault();
+        if (target.dataset.active === "true") {
+          formRef.current?.requestSubmit();
+        } else {
+          target.click();
+        }
+        return;
+      }
+
+      e.preventDefault();
+      if (target === filterInputRef.current) {
+        // Resolve the filter to a project only when the current pick isn't
+        // one of the matches: a filter typed past it means the operator
+        // moved on, while a pick that still matches keeps its worktree choice.
+        const stillMatches =
+          selectedGroup != null && visibleGroups.some((g) => g.main.path === selectedGroup.main.path);
+        if (!stillMatches && visibleGroups[0]) selectProject(visibleGroups[0]);
+      }
+      formRef.current?.requestSubmit();
+    },
+    [selectedGroup, visibleGroups, selectProject],
+  );
+
+  return (
+    <form
+      ref={formRef}
+      onSubmit={onSubmit}
+      onKeyDown={onFormKeyDown}
+      className="mx-auto flex w-full max-w-[60ch] flex-col gap-sp-2 overflow-auto p-sp-3"
+    >
+      <div className="flex flex-col gap-sp-2">
+        <span className="text-fs-13 text-ink">Harness</span>
+        <div className="grid grid-cols-[repeat(auto-fill,minmax(120px,1fr))] gap-sp-2">
+          {presets.map((p) => {
+            const isCustom = p.command === "";
+            const toolId = presetToolId(p);
+            const icon = isCustom ? null : toolId ? harnessIcon(toolId) : harnessIcon("shell");
+            const label = TOOL_LABELS[toolId ?? ""] ?? p.label;
+            const active = p.id === selectedPresetId;
+            return (
+              <button
+                key={p.id}
+                type="button"
+                className={`flex flex-col items-center gap-sp-2 rounded-r-2 border px-sp-2 py-sp-3 text-center ${
+                  active
+                    ? "border-accent bg-accent-tint"
+                    : isCustom
+                      ? "border-dashed border-border text-muted"
+                      : "border-border bg-surface-2 hover:border-border-strong"
+                }`}
+                onClick={() => selectPreset(p.id)}
+              >
+                <span
+                  className={`flex h-8 w-8 items-center justify-center ${active ? "text-ink" : "text-muted"}`}
+                  aria-hidden={icon ? undefined : true}
+                  dangerouslySetInnerHTML={icon ? { __html: icon } : undefined}
+                >
+                  {icon ? undefined : "…"}
+                </span>
+                <span className="max-w-full truncate text-fs-13 font-semibold">
+                  {isCustom ? "Custom…" : label}
+                </span>
+              </button>
+            );
+          })}
+        </div>
+        <input
+          ref={customCommandRef}
+          className="w-full rounded-r-2 border border-border bg-surface-2 px-sp-3 py-2.5 font-mono text-fs-15 text-ink"
+          name="command_raw"
+          type="text"
+          hidden={!isCustomSelected}
+          placeholder="bash -l"
+          autoComplete="off"
+          value={customCommand}
+          onChange={(e) => {
+            const v = e.currentTarget.value;
+            setCustomCommand(v);
+            if (isCustomSelected) {
+              setCommandTouched(true);
+              setCommand(v.trim());
+            }
+          }}
+        />
+      </div>
+
+      <div className="flex flex-col gap-sp-2">
+        <span className="text-fs-13 text-ink">Project</span>
+        <input
+          ref={filterInputRef}
+          className="w-full rounded-r-2 border border-border bg-surface-2 px-sp-3 py-2.5 font-mono text-fs-15 text-ink"
+          type="text"
+          placeholder="Filter projects…"
+          autoComplete="off"
+          value={filterQuery}
+          onChange={(e) => setFilterQuery(e.currentTarget.value)}
+        />
+        {!ready ? (
+          <p className="m-0 px-sp-3 py-1.5 text-fs-13 text-faint">Loading projects…</p>
+        ) : (
+          <div className="grid max-h-[300px] grid-cols-2 gap-sp-2 overflow-y-auto p-px">
+            {visibleGroups.map((group) => {
+              const active = selectedGroup?.main.path === group.main.path;
+              return (
+                <button
+                  key={group.main.path}
+                  type="button"
+                  data-picker="project"
+                  data-active={active || undefined}
+                  className={`flex min-w-0 flex-col gap-0.5 rounded-r-2 border px-sp-3 py-sp-2 text-left ${
+                    active ? "border-accent bg-accent-tint" : "border-border bg-surface-2 hover:border-border-strong"
+                  } ${isStale(group.main, new Date()) ? "opacity-55" : ""}`}
+                  onClick={() => selectProject(group)}
+                >
+                  <span className="truncate text-fs-13 font-semibold text-ink">{group.main.name}</span>
+                  <span className="break-words font-mono text-fs-12 text-muted">
+                    {abbreviateCwd(parentDir(group.main.path), homeDir)}
+                  </span>
+                </button>
+              );
+            })}
+            {discovered.length > 0 && visibleGroups.length === 0 && (
+              <p className="col-span-2 m-0 px-sp-3 py-1.5 text-fs-13 text-faint">No projects match.</p>
+            )}
+          </div>
+        )}
+        <button
+          type="button"
+          className="self-start bg-transparent px-0 py-1 text-fs-13 italic text-muted hover:text-ink"
+          onClick={() => showRawCwd(cwd || homeDir)}
+        >
+          Other directory…
+        </button>
+        {cwdRawVisible && (
+          <DirAutocomplete
+            inputRef={rawCwdRef}
+            className="w-full rounded-r-2 border border-border bg-surface-2 px-sp-3 py-2.5 font-mono text-fs-15 text-ink"
+            name="cwd_raw"
+            type="text"
+            placeholder="/home/user/project"
+            value={cwd}
+            onChange={onRawCwdChange}
+          />
+        )}
+      </div>
+
+      <div className="flex flex-col border-t border-border pt-sp-2">
+        <button
+          type="button"
+          className="flex w-full items-center gap-sp-1 py-sp-2 text-left text-fs-11 text-muted"
+          aria-expanded={!advancedCollapsed}
+          onClick={() => setAdvancedCollapsed((c) => !c)}
+        >
+          <span className={`inline-block transition-transform ${advancedCollapsed ? "-rotate-90" : ""}`}>▾</span>
+          <span className="font-semibold">Advanced</span>
+          <span className="ml-auto text-fs-12 text-muted">{wtEnabled ? "creates a worktree" : "no worktree"}</span>
+        </button>
+        {!advancedCollapsed && (
+          <div className="flex flex-col gap-sp-3 py-sp-2">
+            <label className="flex flex-col gap-1 text-fs-12 text-muted">
+              Session name
+              <input
+                name="name"
+                type="text"
+                autoComplete="off"
+                placeholder="named after your first prompt"
+                className="w-full rounded-r-2 border border-border bg-surface-2 px-sp-3 py-2.5 font-mono text-fs-15 text-ink"
+                value={name}
+                onChange={(e) => setName(e.currentTarget.value)}
+              />
+            </label>
+
+            {structuredSupported && (
+              <label className="flex cursor-pointer items-center gap-sp-2 text-fs-14 text-ink">
+                <input
+                  type="checkbox"
+                  data-role="toggle"
+                  className="size-[18px] accent-accent"
+                  checked={structuredView}
+                  onChange={(e) => {
+                    setStructuredTouched(true);
+                    setStructuredView(e.currentTarget.checked);
+                  }}
+                />
+                Structured view
+              </label>
+            )}
+
+            {selectedGroup && (
+              <div className="flex flex-col gap-sp-2">
+                <span className="text-fs-13 text-ink">Worktree</span>
+                <div className="flex max-h-[200px] flex-col gap-sp-1 overflow-y-auto">
+                  <button
+                    type="button"
+                    data-picker="worktree"
+                    data-active={worktreeChoice === "new" || undefined}
+                    className={pickerButtonClass(worktreeChoice === "new")}
+                    onClick={() => applyWorktreeChoice(selectedGroup, "new")}
+                  >
+                    <span className="text-fs-13 font-semibold text-accent">+ new</span>
+                    <span className="text-right text-fs-12 text-muted">
+                      new worktree of {selectedGroup.main.name}
+                    </span>
+                  </button>
+                  {worktreesOf(selectedGroup).map((wt) => (
+                    <button
+                      key={wt.path}
+                      type="button"
+                      data-picker="worktree"
+                      data-active={worktreeChoice === wt.path || undefined}
+                      className={pickerButtonClass(worktreeChoice === wt.path)}
+                      onClick={() => applyWorktreeChoice(selectedGroup, wt.path)}
+                    >
+                      <span className="text-fs-13 font-semibold text-ink">{wt.name}</span>
+                      <span className="text-right text-fs-12 text-muted">{wt.path}</span>
+                    </button>
+                  ))}
+                </div>
+              </div>
+            )}
+
+            <BranchPicker
+              enabled={wtEnabled}
+              onToggleEnabled={onWtEnabledChange}
+              hint={wtHint}
+              baseRepo={inferredBaseRepo}
+              mode={branchMode}
+              onModeChange={setBranchMode}
+              newBranch={newBranch}
+              onNewBranchChange={(v) => {
+                setBranchEdited(true);
+                setNewBranch(v);
+              }}
+              branches={branchesForCwd}
+              existingBranch={effectiveExistingBranch}
+              onExistingBranchChange={setExistingBranch}
+            />
+          </div>
+        )}
+      </div>
+
+      <p role="alert" aria-live="polite" className={`m-0 text-fs-13 text-danger ${error ? "rounded-r-2 bg-danger-tint px-sp-3 py-sp-2" : "min-h-[1.2em]"}`}>
+        {error}
+      </p>
+      <div className="flex justify-end">
+        <button
+          type="submit"
+          disabled={submitting}
+          className="rounded-r-1 bg-accent px-sp-4 py-sp-2 text-fs-13 font-semibold text-accent-ink disabled:opacity-60"
+        >
+          Spawn
+        </button>
+      </div>
+    </form>
+  );
+}

web/src/components/spawn/SpawnModal.test.tsx

@@ -0,0 +1,108 @@
+// @vitest-environment happy-dom
+import { cleanup, fireEvent, render, screen, waitFor } from "@testing-library/react";
+import { afterEach, beforeEach, describe, expect, it, vi } from "vitest";
+
+import type { DaemonInfo, Project, Session } from "../../types.js";
+import { SpawnModal } from "./SpawnModal.js";
+
+vi.mock("../../api.js", () => ({
+  createSession: vi.fn(),
+  fetchBranches: vi.fn(),
+  fetchNames: vi.fn(),
+  getDaemonInfo: vi.fn(),
+  gitRoot: vi.fn(),
+  listDir: vi.fn(),
+  projects: vi.fn(),
+}));
+
+import { createSession, fetchNames, getDaemonInfo, listDir, projects as fetchProjects } from "../../api.js";
+
+function info(): DaemonInfo {
+  return { hostname: "h", version: "1", home_dir: "/home/n", presets: [{ id: "claude", label: "Claude", command: "claude" }] };
+}
+
+function project(over: Partial<Project> = {}): Project {
+  return { path: "/repos/p", name: "p", ...over };
+}
+
+beforeEach(() => {
+  vi.mocked(getDaemonInfo).mockResolvedValue(info());
+  vi.mocked(fetchProjects).mockResolvedValue([project()]);
+  vi.mocked(fetchNames).mockResolvedValue({ names: ["amber-fox"] });
+  vi.mocked(listDir).mockResolvedValue({ path: "", entries: [] });
+});
+
+afterEach(() => {
+  cleanup();
+  vi.clearAllMocks();
+});
+
+async function settle() {
+  await waitFor(() => expect(screen.queryByText("Loading projects…")).toBeNull());
+}
+
+describe("SpawnModal", () => {
+  it("mounts the spawn form inside a dialog", async () => {
+    render(<SpawnModal onSuccess={vi.fn()} onClose={vi.fn()} />);
+    await settle();
+
+    expect(screen.getByRole("dialog")).not.toBeNull();
+    expect(screen.getByRole("button", { name: "Spawn" })).not.toBeNull();
+  });
+
+  it("titles a seeded open as a clone", async () => {
+    render(<SpawnModal seed={{ command: "claude" }} onSuccess={vi.fn()} onClose={vi.fn()} />);
+    await settle();
+
+    expect(screen.getByText("Spawn similar")).not.toBeNull();
+  });
+
+  it("titles a plain open as a new session", async () => {
+    render(<SpawnModal onSuccess={vi.fn()} onClose={vi.fn()} />);
+    await settle();
+
+    expect(screen.getByText("Spawn session")).not.toBeNull();
+  });
+
+  it("closes on Escape, on Cancel and on a backdrop click", async () => {
+    const onClose = vi.fn();
+    const { unmount } = render(<SpawnModal onSuccess={vi.fn()} onClose={onClose} />);
+    await settle();
+
+    fireEvent.keyDown(screen.getByRole("dialog"), { key: "Escape" });
+    expect(onClose).toHaveBeenCalledTimes(1);
+
+    fireEvent.click(screen.getByRole("button", { name: "Cancel" }));
+    expect(onClose).toHaveBeenCalledTimes(2);
+    unmount();
+
+    render(<SpawnModal onSuccess={vi.fn()} onClose={onClose} />);
+    await settle();
+    fireEvent.click(document.querySelector(".modal-backdrop")!);
+    expect(onClose).toHaveBeenCalledTimes(3);
+  });
+
+  it("closes before handing the created session to its caller", async () => {
+    const created: Session = {
+      id: "s1",
+      cwd: "/repos/p",
+      command: "claude",
+      env: {},
+      metadata: {},
+      status: "Running",
+      created_at: "2026-01-01T00:00:00Z",
+    };
+    vi.mocked(createSession).mockResolvedValue(created);
+    const calls: string[] = [];
+    const onClose = vi.fn(() => calls.push("close"));
+    const onSuccess = vi.fn(() => calls.push("success"));
+    render(<SpawnModal onSuccess={onSuccess} onClose={onClose} />);
+    await settle();
+    fireEvent.click(screen.getByText("p"));
+
+    fireEvent.click(screen.getByRole("button", { name: "Spawn" }));
+
+    await waitFor(() => expect(onSuccess).toHaveBeenCalledWith(created));
+    expect(calls).toEqual(["close", "success"]);
+  });
+});

web/src/components/spawn/SpawnModal.tsx

@@ -0,0 +1,82 @@
+// React port of ../../spawnModal.ts: dialog chrome around SpawnForm. Escape,
+// the ✕ and a click on the backdrop all close it; SpawnForm's own DirAutocomplete
+// swallows Escape first, so dismissing a dropdown never also discards the form.
+
+import { useEffect, useRef } from "react";
+import { createPortal } from "react-dom";
+
+import type { Session } from "../../types.js";
+import { SpawnForm, type SpawnFormProps } from "./SpawnForm.js";
+
+export interface SpawnModalProps {
+  seed?: SpawnFormProps["seed"];
+  /** Called with the created session after a successful submit — the modal
+   * has already closed by then, so the caller decides where it lands. */
+  onSuccess(session: Session): void;
+  onClose(): void;
+}
+
+export function SpawnModal({ seed, onSuccess, onClose }: SpawnModalProps) {
+  const filterInputRef = useRef<HTMLInputElement>(null);
+
+  const handleSuccess = (session: Session) => {
+    onClose();
+    onSuccess(session);
+  };
+
+  // The project filter owns the keyboard on open: typing narrows the list and
+  // Tab from here reaches the harness tiles, matching the TUI's draft pane.
+  useEffect(() => {
+    const host = filterInputRef.current;
+    host?.querySelector<HTMLInputElement>("input[placeholder='Filter projects…']")?.focus();
+  }, []);
+
+  return createPortal(
+    <div
+      className="modal-backdrop fixed inset-0 z-modal flex items-center justify-center bg-bg/70 backdrop-blur-sm"
+      onClick={(e) => {
+        if (e.target === e.currentTarget) onClose();
+      }}
+    >
+      <div
+        role="dialog"
+        aria-modal="true"
+        aria-labelledby="spawn-title"
+        className="flex max-h-[92vh] w-full max-w-[560px] flex-col rounded-r-3 border border-border bg-surface shadow-2"
+        onKeyDown={(e) => {
+          if (e.key === "Escape" && !e.defaultPrevented) {
+            e.preventDefault();
+            onClose();
+          }
+        }}
+      >
+        <header className="flex items-center justify-between border-b border-border p-sp-3">
+          <h2 id="spawn-title" className="m-0 text-fs-15 font-semibold text-ink">
+            {seed ? "Spawn similar" : "Spawn session"}
+          </h2>
+          <button
+            type="button"
+            aria-label="Close"
+            className="rounded-r-1 px-sp-2 py-1 text-fs-15 text-muted hover:text-ink"
+            onClick={onClose}
+          >
+            ✕
+          </button>
+        </header>
+        <div ref={filterInputRef} className="min-h-0 flex-1 overflow-auto">
+          <SpawnForm seed={seed} onSuccess={handleSuccess} />
+        </div>
+        <div className="flex justify-end gap-sp-2 border-t border-border p-sp-3">
+          <button
+            type="button"
+            className="rounded-r-1 border border-border px-sp-3 py-sp-1 text-fs-12 text-ink"
+            onClick={onClose}
+          >
+            Cancel
+          </button>
+        </div>
+      </div>
+    </div>,
+    document.body,
+  );
+}

web/src/components/spawn/index.ts

@@ -0,0 +1,8 @@
+export { SpawnModal, type SpawnModalProps } from "./SpawnModal.js";
+export { SpawnForm, type SpawnFormProps } from "./SpawnForm.js";
+export { DirAutocomplete, type DirAutocompleteProps } from "./DirAutocomplete.js";
+export { BranchPicker, type BranchPickerProps } from "./BranchPicker.js";
+export { useFastForward, type UseFastForwardOptions } from "./useFastForward.js";
+// spawnSimilar.ts is pure seed-building logic with no DOM dependency — reused
+// as-is rather than re-implemented for React (#248).
+export { spawnSimilarSeed, spawnSimilarProjectSeed } from "../../spawnSimilar.js";

web/src/components/spawn/useFastForward.test.ts

@@ -0,0 +1,114 @@
+// @vitest-environment happy-dom
+import { act, renderHook } from "@testing-library/react";
+import { afterEach, describe, expect, it, vi } from "vitest";
+
+import type { Project, Session, SpawnPreset } from "../../types.js";
+import { useFastForward } from "./useFastForward.js";
+
+vi.mock("../../api.js", () => ({
+  createSession: vi.fn(),
+  fetchNames: vi.fn(),
+  getDaemonInfo: vi.fn(),
+  projects: vi.fn(),
+}));
+
+import { createSession, fetchNames, getDaemonInfo, projects as fetchProjects } from "../../api.js";
+
+const mockCreateSession = vi.mocked(createSession);
+const mockFetchNames = vi.mocked(fetchNames);
+const mockGetDaemonInfo = vi.mocked(getDaemonInfo);
+const mockFetchProjects = vi.mocked(fetchProjects);
+
+const presets: SpawnPreset[] = [{ id: "claude", label: "Claude", command: "claude" }];
+
+function project(over: Partial<Project> = {}): Project {
+  return { path: "/repos/p", name: "p", base_repo: "/repos/p", last_command: "claude", ...over };
+}
+
+function session(): Session {
+  return {
+    id: "s1",
+    cwd: "/repos/p",
+    command: "claude",
+    env: {},
+    metadata: {},
+    status: "Running",
+    created_at: "2026-01-01T00:00:00Z",
+  };
+}
+
+afterEach(() => vi.clearAllMocks());
+
+describe("useFastForward", () => {
+  it("spawns immediately once every default resolves", async () => {
+    mockFetchProjects.mockResolvedValue([project()]);
+    mockFetchNames.mockResolvedValue({ names: ["amber-fox"] });
+    mockGetDaemonInfo.mockResolvedValue({ hostname: "h", version: "1", presets });
+    const created = session();
+    mockCreateSession.mockResolvedValue(created);
+    const onSpawned = vi.fn();
+    const onFallback = vi.fn();
+
+    const { result } = renderHook(() => useFastForward({ onSpawned, onFallback }));
+    await act(() => result.current());
+
+    expect(mockCreateSession).toHaveBeenCalledWith(
+      expect.objectContaining({
+        cwd: "/repos/p",
+        command: "claude",
+        worktree: { base_repo: "/repos/p", branch: "amber-fox", create_branch: true },
+      }),
+    );
+    expect(onSpawned).toHaveBeenCalledWith(created);
+    expect(onFallback).not.toHaveBeenCalled();
+  });
+
+  it("falls back to a pre-filled seed when the daemon rejects the spawn", async () => {
+    mockFetchProjects.mockResolvedValue([project()]);
+    mockFetchNames.mockResolvedValue({ names: ["amber-fox"] });
+    mockGetDaemonInfo.mockResolvedValue({ hostname: "h", version: "1", presets });
+    mockCreateSession.mockRejectedValue(new Error("boom"));
+    const onSpawned = vi.fn();
+    const onFallback = vi.fn();
+
+    const { result } = renderHook(() => useFastForward({ onSpawned, onFallback }));
+    await act(() => result.current());
+
+    expect(onSpawned).not.toHaveBeenCalled();
+    expect(onFallback).toHaveBeenCalledWith(
+      expect.objectContaining({ cwd: "/repos/p", command: "claude", worktreeBranch: "amber-fox" }),
+    );
+  });
+
+  it("falls back when nothing is known yet (no projects, no handle)", async () => {
+    mockFetchProjects.mockResolvedValue([]);
+    mockFetchNames.mockRejectedValue(new Error("exhausted"));
+    mockGetDaemonInfo.mockResolvedValue({ hostname: "h", version: "1", presets });
+    const onSpawned = vi.fn();
+    const onFallback = vi.fn();
+
+    const { result } = renderHook(() => useFastForward({ onSpawned, onFallback }));
+    await act(() => result.current());
+
+    expect(mockCreateSession).not.toHaveBeenCalled();
+    expect(onFallback).toHaveBeenCalledWith(expect.objectContaining({ cwd: "" }));
+  });
+
+  it("prefers the focused session's project over the freshest one", async () => {
+    mockFetchProjects.mockResolvedValue([
+      project({ path: "/repos/fresh", name: "fresh" }),
+      project({ path: "/repos/current", name: "current", last_activity: "2020-01-01T00:00:00Z" }),
+    ]);
+    mockFetchNames.mockResolvedValue({ names: ["h"] });
+    mockGetDaemonInfo.mockResolvedValue({ hostname: "h", version: "1", presets });
+    mockCreateSession.mockResolvedValue(session());
+    const onSpawned = vi.fn();
+
+    const { result } = renderHook(() =>
+      useFastForward({ focusedCwd: "/repos/current", onSpawned, onFallback: vi.fn() }),
+    );
+    await act(() => result.current());
+
+    expect(mockCreateSession).toHaveBeenCalledWith(expect.objectContaining({ cwd: "/repos/current" }));
+  });
+});

web/src/components/spawn/useFastForward.ts

@@ -0,0 +1,57 @@
+// React port of main.ts's `requestFastForward` (#67): one keystroke, every
+// spawn default resolved — most recent project (or the one the operator is
+// on), its last command, a fresh generated branch handle, worktree on with a
+// new branch. A failed POST, or any missing default, falls back to the same
+// pre-filled form `onFallback` opens, rather than a dead keystroke.
+
+import { useCallback } from "react";
+
+import { createSession, fetchNames, getDaemonInfo, projects as fetchProjects } from "../../api.js";
+import { buildFastForwardRequest, fastForwardSeed, resolveFastForward } from "../../fastForward.js";
+import { FALLBACK_PRESETS, type SpawnFormSeed } from "../../spawnForm.js";
+import type { Project, Session, SpawnPreset } from "../../types.js";
+
+export interface UseFastForwardOptions {
+  /** The project of the session the operator is currently on, if any — its
+   * base repo for a worktree session, else its cwd. Same rule the plain
+   * spawn modal's seed uses, so `N` lands where the operator currently is. */
+  focusedCwd?: string;
+  onSpawned(session: Session): void;
+  onFallback(seed: SpawnFormSeed): void;
+}
+
+async function resolvePresets(): Promise<SpawnPreset[]> {
+  try {
+    const info = await getDaemonInfo();
+    if (info.presets?.length) return info.presets;
+  } catch {
+    // keep fallbacks
+  }
+  return FALLBACK_PRESETS;
+}
+
+/** Returns a function to call on the fast-forward keystroke. */
+export function useFastForward(opts: UseFastForwardOptions): () => Promise<void> {
+  const { focusedCwd, onSpawned, onFallback } = opts;
+
+  return useCallback(async () => {
+    const [discovered, handle] = await Promise.all([
+      fetchProjects().catch(() => [] as Project[]),
+      fetchNames(1)
+        .then((batch) => batch.names[0])
+        .catch(() => undefined),
+    ]);
+    const presets = await resolvePresets();
+    const resolved = resolveFastForward(discovered, presets, handle, focusedCwd);
+    const req = buildFastForwardRequest(resolved);
+    if (req) {
+      try {
+        onSpawned(await createSession(req));
+        return;
+      } catch {
+        // degrades to the pre-filled form below, never a silent dead key
+      }
+    }
+    onFallback(fastForwardSeed(resolved));
+  }, [focusedCwd, onSpawned, onFallback]);
+}

web/src/spawnForm.ts

@@ -15,9 +15,9 @@ import { splitCommand, toolIconSvg, TOOL_LABELS } from "./toolIcon.js";
 import { abbreviateCwd } from "./types.js";
 import type { BranchInfo, CreateSessionRequest, Project, Session, SpawnPreset } from "./types.js";
 
-const STRUCTURED_VIEW_TOOLS = new Set(["claude", "codex"]);
+export const STRUCTURED_VIEW_TOOLS = new Set(["claude", "codex"]);
 
-function supportsStructuredView(command: string): boolean {
+export function supportsStructuredView(command: string): boolean {
   const tool = splitCommand(command).tool;
   return tool !== null && STRUCTURED_VIEW_TOOLS.has(tool);
 }

web/src/stores/layoutStore.test.ts

@@ -0,0 +1,162 @@
+import { describe, expect, it, vi } from "vitest";
+
+import { leaves } from "../layout.js";
+import { createLayoutStore, type LayoutStoreOptions } from "./layoutStore.js";
+
+function fakeScheduler() {
+  const timers = new Map<number, () => void>();
+  let id = 0;
+  return {
+    scheduler: {
+      setTimer: (fn: () => void) => {
+        const h = ++id;
+        timers.set(h, fn);
+        return h;
+      },
+      clearTimer: (h: number) => {
+        timers.delete(h);
+      },
+    },
+    flush() {
+      for (const fn of [...timers.values()]) fn();
+      timers.clear();
+    },
+  };
+}
+
+function store(opts: Partial<LayoutStoreOptions> = {}) {
+  return createLayoutStore({
+    load: () => null,
+    save: vi.fn(),
+    scheduler: fakeScheduler().scheduler,
+    ...opts,
+  });
+}
+
+describe("layoutStore", () => {
+  it("boots with a single empty tab when nothing is persisted", () => {
+    const { layout } = store().getState();
+    expect(layout.tabs).toHaveLength(1);
+    expect(layout.tabs[0].root).toBeNull();
+  });
+
+  it("boots from a persisted layout when one loads", () => {
+    const persisted = store().getState().layout;
+    persisted.tabs[0].title = "Restored";
+    const s = store({ load: () => persisted });
+    expect(s.getState().layout.tabs[0].title).toBe("Restored");
+  });
+
+  it("openSession seeds the active tab from empty", () => {
+    const s = store();
+    s.getState().openSession("s1");
+    expect(leaves(s.getState().layout.tabs[0].root).map((l) => l.sessionId)).toEqual(["s1"]);
+  });
+
+  it("openSession focuses an already-open session instead of moving it", () => {
+    const s = store();
+    s.getState().openSession("s1");
+    s.getState().openSessionBeside("s2");
+    s.getState().openSession("s1");
+    const ids = leaves(s.getState().layout.tabs[0].root)
+      .map((l) => l.sessionId)
+      .sort();
+    expect(ids).toEqual(["s1", "s2"]);
+  });
+
+  it("openSessionBeside splits next to the focused pane instead of replacing it", () => {
+    const s = store();
+    s.getState().openSession("s1");
+    s.getState().openSessionBeside("s2");
+    expect(leaves(s.getState().layout.tabs[0].root)).toHaveLength(2);
+  });
+
+  it("openSessionBeside(newTab) opens its own tab", () => {
+    const s = store();
+    s.getState().openSession("s1");
+    s.getState().openSessionBeside("s2", { newTab: true });
+    expect(s.getState().layout.tabs).toHaveLength(2);
+    expect(leaves(s.getState().layout.tabs[1].root).map((l) => l.sessionId)).toEqual(["s2"]);
+  });
+
+  it("closeSession removes the pane and closes a tab left with nothing else", () => {
+    const s = store();
+    s.getState().openSession("s1");
+    s.getState().addTab();
+    s.getState().openSession("s2");
+    s.getState().closeSession("s2");
+    expect(s.getState().layout.tabs).toHaveLength(1);
+    expect(leaves(s.getState().layout.tabs[0].root).map((l) => l.sessionId)).toEqual(["s1"]);
+  });
+
+  it("splitFocused adds a leaf beside the given one and focuses it", () => {
+    const s = store();
+    s.getState().openSession("s1");
+    const first = leaves(s.getState().layout.tabs[0].root)[0].id;
+    s.getState().splitFocused(first, "row");
+    const tab = s.getState().layout.tabs[0];
+    expect(leaves(tab.root)).toHaveLength(2);
+    expect(tab.focusedLeafId).not.toBe(first);
+  });
+
+  it("closePane collapses the split back to the surviving leaf", () => {
+    const s = store();
+    s.getState().openSession("s1");
+    const first = leaves(s.getState().layout.tabs[0].root)[0].id;
+    s.getState().splitFocused(first, "row");
+    s.getState().closePane(first);
+    const remaining = leaves(s.getState().layout.tabs[0].root);
+    expect(remaining).toHaveLength(1);
+    expect(remaining[0].sessionId).toBeNull();
+  });
+
+  it("addTab/selectTab/closeTab manage the tab strip", () => {
+    const s = store();
+    s.getState().addTab();
+    expect(s.getState().layout.tabs).toHaveLength(2);
+    const [first, second] = s.getState().layout.tabs;
+    s.getState().selectTab(first.id);
+    expect(s.getState().layout.activeTabId).toBe(first.id);
+    s.getState().closeTab(second.id);
+    expect(s.getState().layout.tabs).toHaveLength(1);
+  });
+
+  it("reorderTab moves a tab to sit where another one is", () => {
+    const s = store();
+    s.getState().addTab();
+    s.getState().addTab();
+    const [a, b, c] = s.getState().layout.tabs;
+    s.getState().reorderTab(c.id, a.id);
+    expect(s.getState().layout.tabs.map((t) => t.id)).toEqual([c.id, a.id, b.id]);
+  });
+
+  it("reconcile drops leaves for sessions no longer live, and closes a tab left with nothing else", () => {
+    const s = store();
+    s.getState().openSession("s1");
+    s.getState().addTab();
+    s.getState().openSession("dead");
+    s.getState().reconcile(new Set(["s1"]));
+    const { layout } = s.getState();
+    expect(layout.tabs).toHaveLength(1);
+    expect(leaves(layout.tabs[0].root).map((l) => l.sessionId)).toEqual(["s1"]);
+  });
+
+  it("reconcile leaves an intentionally empty tab alone", () => {
+    const s = store();
+    s.getState().addTab();
+    s.getState().reconcile(new Set());
+    expect(s.getState().layout.tabs).toHaveLength(2);
+  });
+
+  it("debounces persistence to a single write once idle", () => {
+    const save = vi.fn();
+    const { scheduler, flush } = fakeScheduler();
+    const s = store({ save, scheduler });
+    s.getState().openSession("s1");
+    s.getState().openSessionBeside("s2");
+    expect(save).not.toHaveBeenCalled();
+    flush();
+    expect(save).toHaveBeenCalledTimes(1);
+    expect(save.mock.calls[0][0]).toBe(s.getState().layout);
+  });
+});

web/src/stores/layoutStore.ts

@@ -0,0 +1,253 @@
+// Zustand wrapper around ../layout.js's layout tree model (issue #249).
+// Actions mutate the tree in place via layout.js's own functions, then
+// re-wrap it so `useStore` selectors relying on referential equality notice.
+
+import { create } from "zustand";
+
+import {
+  activeTab,
+  addTab as addTabToLayout,
+  closeTab as closeTabInLayout,
+  emptyLayout,
+  equalizeRatios,
+  findLeaf,
+  leaves,
+  locateSession,
+  removeLeaf,
+  splitLeaf,
+  type Dir,
+  type Layout,
+} from "../layout.js";
+import { loadLayout, saveLayout } from "../persistence.js";
+import type { Uuid } from "../types.js";
+import type { Scheduler } from "./catalogStore.js";
+
+const SAVE_DEBOUNCE_MS = 400;
+
+const browserScheduler: Scheduler = {
+  setTimer: (fn, ms) => window.setTimeout(fn, ms),
+  clearTimer: (h) => window.clearTimeout(h),
+};
+
+export interface LayoutStoreOptions {
+  load?: () => Layout | null;
+  save?: (layout: Layout) => void;
+  scheduler?: Scheduler;
+}
+
+export interface LayoutState {
+  layout: Layout;
+}
+
+export interface LayoutActions {
+  /** Bring `sessionId` into view: focus it if already open, else fill the
+   *  focused pane (VS-Code-style replace), or seed an empty tab. */
+  openSession(sessionId: Uuid): void;
+  /** Same, but never evicts a running pane: reuses an empty leaf or splits
+   *  beside the focused one. `newTab` opens its own tab instead. */
+  openSessionBeside(sessionId: Uuid, opts?: { newTab?: boolean }): void;
+  /** Drop the pane showing `sessionId` (closed/killed elsewhere). */
+  closeSession(sessionId: Uuid): void;
+  splitFocused(leafId: string, dir: Dir): void;
+  closePane(leafId: string): void;
+  focusLeaf(leafId: string): void;
+  selectTab(tabId: string): void;
+  addTab(): void;
+  closeTab(tabId: string): void;
+  /** Tab-strip drag-and-drop reordering: moves `fromId` to sit where `toId`
+   *  currently is. */
+  reorderTab(fromId: string, toId: string): void;
+  /** Persist whatever a divider drag left on a split's `ratio` — the drag
+   *  itself mutates the node directly (same trade as the vanilla version),
+   *  this just re-wraps and schedules the save. */
+  commitLayout(): void;
+  /** Boot reconciliation: drop every leaf whose session isn't in `liveIds`,
+   *  and close any tab that held nothing else. A session killed while its
+   *  tab was closed is pruned instead of leaving a pane that can only ever
+   *  show a dead socket. Tabs that were already empty are left alone. */
+  reconcile(liveIds: ReadonlySet<Uuid>): void;
+}
+
+export type LayoutStore = LayoutState & LayoutActions;
+export type LayoutStoreApi = ReturnType<typeof createLayoutStore>;
+
+/** Builds an isolated layout store — load/save/scheduler are injectable
+ *  (tests), or default to the real localStorage persistence.ts + browser
+ *  timers. Same shape as {@link createCatalogStore}. */
+export function createLayoutStore(opts: LayoutStoreOptions = {}) {
+  const load = opts.load ?? loadLayout;
+  const save = opts.save ?? saveLayout;
+  const scheduler = opts.scheduler ?? browserScheduler;
+
+  let saveTimer: number | null = null;
+
+  return create<LayoutStore>((set, get) => {
+    function touch(): void {
+      // A shallow clone, not the same reference back: `useStore` (and any
+      // other Object.is-based selector) needs the identity to change to
+      // notice a mutation made in place on the tree underneath.
+      set({ layout: { ...get().layout } });
+    }
+
+    function schedulePersist(): void {
+      if (saveTimer !== null) scheduler.clearTimer(saveTimer);
+      saveTimer = scheduler.setTimer(() => {
+        saveTimer = null;
+        save(get().layout);
+      }, SAVE_DEBOUNCE_MS);
+    }
+
+    function commit(): void {
+      touch();
+      schedulePersist();
+    }
+
+    function mutate(fn: (layout: Layout) => void): void {
+      fn(get().layout);
+      commit();
+    }
+
+    return {
+      layout: load() ?? emptyLayout(),
+
+      openSession(sessionId) {
+        mutate((layout) => {
+          const loc = locateSession(layout, sessionId);
+          if (loc) {
+            layout.activeTabId = loc.tab.id;
+            loc.tab.focusedLeafId = loc.leaf.id;
+            return;
+          }
+          const tab = activeTab(layout);
+          if (!tab.root) {
+            tab.focusedLeafId = splitLeaf(tab, "", sessionId, "row", true);
+            return;
+          }
+          const target = (tab.focusedLeafId && findLeaf(tab.root, tab.focusedLeafId)) || leaves(tab.root)[0];
+          target.sessionId = sessionId;
+          tab.focusedLeafId = target.id;
+        });
+      },
+
+      openSessionBeside(sessionId, placement) {
+        mutate((layout) => {
+          const loc = locateSession(layout, sessionId);
+          if (loc) {
+            layout.activeTabId = loc.tab.id;
+            loc.tab.focusedLeafId = loc.leaf.id;
+            return;
+          }
+          if (placement?.newTab) {
+            const fresh = addTabToLayout(layout);
+            fresh.focusedLeafId = splitLeaf(fresh, "", sessionId, "row", true);
+            return;
+          }
+          const tab = activeTab(layout);
+          if (!tab.root || !tab.focusedLeafId || !findLeaf(tab.root, tab.focusedLeafId)) {
+            tab.focusedLeafId = splitLeaf(tab, "", sessionId, "row", true);
+            return;
+          }
+          const target = findLeaf(tab.root, tab.focusedLeafId)!;
+          if (!target.sessionId) {
+            target.sessionId = sessionId;
+          } else {
+            tab.focusedLeafId = splitLeaf(tab, target.id, sessionId, "row", true);
+            equalizeRatios(tab.root);
+          }
+        });
+      },
+
+      closeSession(sessionId) {
+        mutate((layout) => {
+          const loc = locateSession(layout, sessionId);
+          if (!loc) return;
+          removeLeaf(loc.tab, loc.leaf.id);
+          if (loc.tab.focusedLeafId === loc.leaf.id) {
+            loc.tab.focusedLeafId = leaves(loc.tab.root)[0]?.id ?? null;
+          }
+          if (!loc.tab.root) closeTabInLayout(layout, loc.tab.id);
+        });
+      },
+
+      splitFocused(leafId, dir) {
+        mutate((layout) => {
+          const tab = activeTab(layout);
+          tab.focusedLeafId = splitLeaf(tab, leafId, null, dir, true);
+          equalizeRatios(tab.root);
+        });
+      },
+
+      closePane(leafId) {
+        mutate((layout) => {
+          const tab = activeTab(layout);
+          removeLeaf(tab, leafId);
+          if (tab.focusedLeafId === leafId) {
+            tab.focusedLeafId = leaves(tab.root)[0]?.id ?? null;
+          }
+        });
+      },
+
+      focusLeaf(leafId) {
+        mutate((layout) => {
+          const tab = activeTab(layout);
+          if (tab.focusedLeafId === leafId) return;
+          tab.focusedLeafId = leafId;
+        });
+      },
+
+      selectTab(tabId) {
+        mutate((layout) => {
+          if (layout.tabs.some((t) => t.id === tabId)) layout.activeTabId = tabId;
+        });
+      },
+
+      addTab() {
+        mutate((layout) => {
+          addTabToLayout(layout);
+        });
+      },
+
+      closeTab(tabId) {
+        mutate((layout) => {
+          closeTabInLayout(layout, tabId);
+        });
+      },
+
+      reorderTab(fromId, toId) {
+        mutate((layout) => {
+          if (fromId === toId) return;
+          const from = layout.tabs.findIndex((t) => t.id === fromId);
+          const to = layout.tabs.findIndex((t) => t.id === toId);
+          if (from < 0 || to < 0) return;
+          const [moved] = layout.tabs.splice(from, 1);
+          layout.tabs.splice(to, 0, moved);
+        });
+      },
+
+      commitLayout() {
+        commit();
+      },
+
+      reconcile(liveIds) {
+        mutate((layout) => {
+          const emptied: string[] = [];
+          for (const tab of [...layout.tabs]) {
+            let prunedAny = false;
+            for (const leaf of leaves(tab.root)) {
+              if (leaf.sessionId && !liveIds.has(leaf.sessionId)) {
+                removeLeaf(tab, leaf.id);
+                prunedAny = true;
+              }
+            }
+            if (!prunedAny) continue;
+            if (!tab.focusedLeafId || !findLeaf(tab.root, tab.focusedLeafId)) {
+              tab.focusedLeafId = leaves(tab.root)[0]?.id ?? null;
+            }
+            if (!tab.root) emptied.push(tab.id);
+          }
+          for (const id of emptied) closeTabInLayout(layout, id);
+        });
+      },
+    };
+  });
+}