feat(maestro): port preview mode to React (#251)

Extends layoutStore with previewSession/commitPreview, mirroring
multiview.ts's preview controller: a throwaway tab for a session no pane
holds, byte-for-byte restore on end, and every real layout-changing action
drops an active preview first. Never schedules a save while a preview is up.

Adds usePreview, wrapping the store actions with previewSchedule.ts's
120ms leading-edge throttle, plus a touch guard and an immediate `end()`
for real changes. MultiView/TabBar render the preview hue on the pane
titlebar, canvas ring/chip, and the throwaway tab (dashed, unclickable).

web/src/commandBar.ts

@@ -21,7 +21,7 @@ import {
   type MatchRange,
   type MatchResult,
   type SessionCandidate,
-} from "./matcher.js";
+} from "./utils/matcher.js";
 import { splitPath } from "./paths.js";
 import { splitCommand } from "./toolIcon.js";
 import { abbreviateCwd, sessionName, sessionTreePath, type Session } from "./types.js";

web/src/components/commandBar/ActionPage.tsx

@@ -0,0 +1,34 @@
+// A session's action page (docs/COMMAND-BAR.md, "Drill-in"): the same overlay
+// over kill, rename and spawn similar, still fuzzy searchable. The page names
+// the session it acts on, so the target is never ambiguous.
+
+import { sessionName, type Session } from "../../types.js";
+import { ResultList } from "./ResultList.js";
+import type { BarGroup } from "./model.js";
+
+export interface ActionPageProps {
+  session: Session;
+  /** The three actions, ranked by the query line — one kind, so no headers. */
+  groups: BarGroup[];
+  highlighted: number;
+  onActivate(index: number): void;
+}
+
+export function ActionPage({ session, groups, highlighted, onActivate }: ActionPageProps) {
+  return (
+    <>
+      <div
+        role="presentation"
+        className="cmdbar-page-header border-b border-border px-sp-3 py-sp-1 text-fs-11 text-muted"
+      >
+        {`Actions for "${sessionName(session)}"`}
+      </div>
+      <ResultList
+        groups={groups}
+        showHeaders={false}
+        highlighted={highlighted}
+        onActivate={onActivate}
+      />
+    </>
+  );
+}

web/src/components/commandBar/CommandBar.test.tsx

@@ -0,0 +1,407 @@
+// @vitest-environment happy-dom
+import { cleanup, fireEvent, render, screen, waitFor } from "@testing-library/react";
+import { afterEach, describe, expect, it, vi } from "vitest";
+
+import {
+  KILL_SESSION_EVENT,
+  RENAME_SESSION_EVENT,
+  SESSION_SELECT_EVENT,
+  SPAWN_SIMILAR_EVENT,
+} from "../../catalog.js";
+import { COMMAND_SPECS, type Command } from "../../commands.js";
+import type { Session } from "../../types.js";
+import { CommandBar, SPAWN_DIRECTORY_EVENT, type CommandBarProps } from "./CommandBar.js";
+
+vi.mock("../../api.js", async (importOriginal) => ({
+  ...(await importOriginal<Record<string, unknown>>()),
+  listDir: vi.fn(async () => ({ path: "", entries: [] })),
+  projects: vi.fn(async () => []),
+  recentDirs: vi.fn(async () => ({ dirs: [] })),
+}));
+
+function session(id: string, over: Partial<Session> = {}): Session {
+  return {
+    id,
+    cwd: "/home/me/dev/api",
+    command: "bash",
+    env: {},
+    metadata: { name: id },
+    status: "Running",
+    created_at: "2026-01-01T00:00:00Z",
+    ...over,
+  };
+}
+
+function command(over: Partial<Command> = {}): Command {
+  return {
+    id: "cmd",
+    label: "A command",
+    group: "Test",
+    keys: ["a"],
+    scope: "global",
+    run: () => {},
+    ...over,
+  };
+}
+
+const sessions = [
+  session("api", { current_cwd: "/home/me/dev/api", last_focused_at: "2026-01-01T11:00:00Z" }),
+  session("worker", {
+    current_cwd: "/home/me/dev/worker",
+    last_focused_at: "2026-01-01T10:00:00Z",
+  }),
+  session("notes", { current_cwd: "/home/me/notes" }),
+];
+
+function open(over: Partial<CommandBarProps> = {}) {
+  return render(
+    <CommandBar
+      sessions={sessions}
+      homeDir="/home/me"
+      commands={[]}
+      onOpenSession={() => {}}
+      onClose={() => {}}
+      {...over}
+    />,
+  );
+}
+
+function input(): HTMLInputElement {
+  return screen.getByRole("combobox") as HTMLInputElement;
+}
+
+function type(value: string): void {
+  fireEvent.change(input(), { target: { value } });
+}
+
+function press(key: string, init: Record<string, unknown> = {}): void {
+  fireEvent.keyDown(input(), { key, ...init });
+}
+
+function rowNames(): string[] {
+  return [...document.querySelectorAll(".cmdbar-row-name")].map((r) => r.textContent ?? "");
+}
+
+function activeName(): string | undefined {
+  return document.querySelector(".cmdbar-row.is-active .cmdbar-row-name")?.textContent ?? undefined;
+}
+
+afterEach(() => {
+  cleanup();
+  vi.clearAllMocks();
+});
+
+describe("the overlay", () => {
+  it("is a registry command, so the help overlay lists it", () => {
+    const spec = COMMAND_SPECS.find((s) => s.id === "bar.open");
+    expect(spec?.keys).toEqual(["ctrl-k"]);
+    expect(spec?.scope).toBe("global");
+  });
+
+  it("lists sessions by recency under a kind header on an empty query", () => {
+    open();
+    expect([...document.querySelectorAll(".cmdbar-group-title")].map((h) => h.textContent)).toEqual([
+      "Sessions",
+    ]);
+    expect(rowNames()).toEqual(["api", "worker", "notes"]);
+    expect(activeName()).toBe("api");
+  });
+
+  it("ranks a typed query through the matcher and highlights the matched range", () => {
+    open();
+    type("wor");
+    expect(rowNames()).toEqual(["worker"]);
+    expect([...document.querySelectorAll(".cmdbar-row-name mark")].map((m) => m.textContent)).toEqual(
+      ["wor"],
+    );
+  });
+
+  it("highlights inside the directory when that is the field that matched", () => {
+    open();
+    type("dev/wor");
+    expect(rowNames()).toEqual(["worker"]);
+    expect(
+      [...document.querySelectorAll(".cmdbar-row-meta mark")].map((m) => m.textContent).join(""),
+    ).toBe("dev/wor");
+  });
+
+  it("renders a no-results state rather than an empty list", () => {
+    open();
+    type("zzz");
+    expect(rowNames()).toEqual([]);
+    expect(screen.getByText("No matches")).toBeDefined();
+  });
+
+  it("moves the highlight with the arrows and ctrl-p/ctrl-n, without wrapping", () => {
+    open();
+    press("ArrowUp");
+    expect(activeName()).toBe("api");
+    press("ArrowDown");
+    expect(activeName()).toBe("worker");
+    press("n", { ctrlKey: true });
+    expect(activeName()).toBe("notes");
+    press("ArrowDown");
+    expect(activeName()).toBe("notes");
+    press("p", { ctrlKey: true });
+    expect(activeName()).toBe("worker");
+  });
+
+  it("puts the highlight back on the best row whenever the query changes", () => {
+    open();
+    press("ArrowDown");
+    expect(activeName()).toBe("worker");
+    type("o");
+    expect(activeName()).toBe(rowNames()[0]);
+  });
+
+  it("opens the highlighted session through the caller's open path on Enter", () => {
+    const onOpenSession = vi.fn();
+    const onClose = vi.fn();
+    open({ onOpenSession, onClose });
+    press("ArrowDown");
+    press("Enter");
+    expect(onOpenSession).toHaveBeenCalledWith("worker");
+    expect(onClose).toHaveBeenCalled();
+  });
+
+  it("raises the shared select event when the caller supplies no handler", () => {
+    const listener = vi.fn();
+    document.addEventListener(SESSION_SELECT_EVENT, listener);
+    open({ onOpenSession: undefined });
+    press("Enter");
+    document.removeEventListener(SESSION_SELECT_EVENT, listener);
+    expect((listener.mock.calls[0][0] as CustomEvent).detail).toBe("api");
+  });
+
+  it("Enter on an empty result list does nothing", () => {
+    const onOpenSession = vi.fn();
+    const onClose = vi.fn();
+    open({ onOpenSession, onClose });
+    type("zzz");
+    press("Enter");
+    expect(onOpenSession).not.toHaveBeenCalled();
+    expect(onClose).not.toHaveBeenCalled();
+  });
+
+  it("closes on Escape and on a press outside the panel", () => {
+    const onClose = vi.fn();
+    open({ onClose });
+    press("Escape");
+    expect(onClose).toHaveBeenCalledTimes(1);
+    fireEvent.mouseDown(document.querySelector(".cmdbar-overlay") as HTMLElement);
+    expect(onClose).toHaveBeenCalledTimes(2);
+  });
+
+  it("stays open for a press inside the panel", () => {
+    const onClose = vi.fn();
+    open({ onClose });
+    fireEvent.mouseDown(document.querySelector(".cmdbar-panel") as HTMLElement);
+    expect(onClose).not.toHaveBeenCalled();
+  });
+
+  it("returns the keyboard to whatever held it when it opened", () => {
+    const before = document.createElement("input");
+    document.body.appendChild(before);
+    before.focus();
+    const view = open();
+    expect(document.activeElement).toBe(input());
+    view.unmount();
+    expect(document.activeElement).toBe(before);
+    before.remove();
+  });
+
+  it("clicking a row opens that session", () => {
+    const onOpenSession = vi.fn();
+    open({ onOpenSession });
+    fireEvent.click(document.querySelectorAll(".cmdbar-row")[1]);
+    expect(onOpenSession).toHaveBeenCalledWith("worker");
+  });
+});
+
+describe("the sigils", () => {
+  const commands = [
+    command({ id: "help.toggle", label: "Show help", keys: ["?"] }),
+    command({ id: "spawn.open", label: "Open the spawn form", keys: ["n"], run: vi.fn() }),
+  ];
+
+  it("lists every command in registry order under `>`, with its chord and no header", () => {
+    open({ commands });
+    type(">");
+    expect(rowNames()).toEqual(["Show help", "Open the spawn form"]);
+    expect(document.querySelectorAll(".cmdbar-group-title")).toHaveLength(0);
+    expect([...document.querySelectorAll(".cmdbar-row-keys kbd")].map((k) => k.textContent)).toEqual(
+      ["?", "n"],
+    );
+  });
+
+  it("matches a command on its label and never on its chord", () => {
+    open({ commands });
+    type(">help");
+    expect(rowNames()).toEqual(["Show help"]);
+    type(">?");
+    expect(rowNames()).toEqual([]);
+  });
+
+  it("runs the highlighted command on Enter", () => {
+    const run = vi.fn();
+    open({ commands: [command({ label: "Run me", run })] });
+    type(">run");
+    press("Enter");
+    expect(run).toHaveBeenCalled();
+  });
+
+  it("keeps the list mixed with no sigil, and returns to it when the sigil goes", () => {
+    open({ commands });
+    type(">show");
+    expect(rowNames()).toEqual(["Show help"]);
+    type("");
+    expect(rowNames()).toEqual(["api", "worker", "notes"]);
+  });
+
+  it("lists recent dirs then projects under `~`", async () => {
+    open({ loadDirectories: async () => ["/home/me/dev/api", "/srv/x"] });
+    type("~");
+    await waitFor(() => expect(rowNames()).toEqual(["~/dev/api", "/srv/x"]));
+    expect(document.querySelectorAll(".cmdbar-group-title")).toHaveLength(0);
+  });
+
+  it("fills the list in from /api/dirs as a path is typed", async () => {
+    const listDirEntries = vi.fn(async () => ["maestro", "api"]);
+    open({ loadDirectories: async () => ["/home/me/notes"], listDirEntries });
+    // The second `~` is a literal query character and reaches the matcher.
+    type("~~/dev/mae");
+    await waitFor(() => expect(rowNames()).toEqual(["~/dev/maestro"]));
+    expect(listDirEntries).toHaveBeenCalledWith("~/dev/");
+  });
+
+  it("spawns in the highlighted directory on Enter", async () => {
+    const onSpawnDirectory = vi.fn();
+    open({ loadDirectories: async () => ["/srv/x"], onSpawnDirectory });
+    type("~");
+    await waitFor(() => expect(rowNames()).toEqual(["/srv/x"]));
+    press("Enter");
+    expect(onSpawnDirectory).toHaveBeenCalledWith("/srv/x");
+  });
+
+  it("raises the spawn event when the caller supplies no handler", async () => {
+    const listener = vi.fn();
+    document.addEventListener(SPAWN_DIRECTORY_EVENT, listener);
+    open({ loadDirectories: async () => ["/srv/x"] });
+    type("~");
+    await waitFor(() => expect(rowNames()).toEqual(["/srv/x"]));
+    press("Enter");
+    document.removeEventListener(SPAWN_DIRECTORY_EVENT, listener);
+    expect((listener.mock.calls[0][0] as CustomEvent).detail).toBe("/srv/x");
+  });
+});
+
+describe("the session action page", () => {
+  function drill(over: Partial<CommandBarProps> = {}) {
+    const view = open(over);
+    press("Tab");
+    return view;
+  }
+
+  it("lists kill, rename and spawn similar for the highlighted session", () => {
+    drill();
+    expect(rowNames()).toEqual(["Kill session", "Rename session", "Spawn similar"]);
+  });
+
+  it("names the session being acted on", () => {
+    drill();
+    expect(screen.getByText('Actions for "api"')).toBeDefined();
+  });
+
+  it("drops the kind headers, since the page is one kind", () => {
+    drill();
+    expect(document.querySelectorAll(".cmdbar-group-title")).toHaveLength(0);
+  });
+
+  it("stays fuzzy-searchable over the three actions", () => {
+    drill();
+    type("ren");
+    expect(rowNames()).toEqual(["Rename session"]);
+  });
+
+  it("does nothing on a command row", () => {
+    open({ commands: [command({ label: "Show help" })] });
+    type(">show");
+    press("Tab");
+    expect(rowNames()).toEqual(["Show help"]);
+  });
+
+  it("does nothing on a directory row", async () => {
+    open({ loadDirectories: async () => ["/srv/x"] });
+    type("~");
+    await waitFor(() => expect(rowNames()).toEqual(["/srv/x"]));
+    press("Tab");
+    expect(rowNames()).toEqual(["/srv/x"]);
+  });
+
+  it("returns to the result list on Escape, with the previous query intact", () => {
+    const onClose = vi.fn();
+    open({ onClose });
+    type("wor");
+    press("Tab");
+    expect(rowNames()).toEqual(["Kill session", "Rename session", "Spawn similar"]);
+    press("Escape");
+    expect(onClose).not.toHaveBeenCalled();
+    expect(input().value).toBe("wor");
+    expect(rowNames()).toEqual(["worker"]);
+  });
+
+  it("returns to the result list on backspace at an empty query", () => {
+    drill();
+    fireEvent.keyDown(input(), { key: "Backspace" });
+    expect(rowNames()).toEqual(["api", "worker", "notes"]);
+  });
+
+  it("keeps backspace ordinary while the page's query has text", () => {
+    drill();
+    type("ren");
+    fireEvent.keyDown(input(), { key: "Backspace" });
+    expect(rowNames()).toEqual(["Rename session"]);
+  });
+
+  it("closes the bar when the page's Escape has already been used", () => {
+    const onClose = vi.fn();
+    drill({ onClose });
+    press("Escape");
+    press("Escape");
+    expect(onClose).toHaveBeenCalledTimes(1);
+  });
+
+  it("raises kill, rename and spawn similar with the session, closing the bar", () => {
+    for (const [query, key] of [
+      ["kill", "onKillSession"],
+      ["rename", "onRenameSession"],
+      ["spawn", "onSpawnSimilar"],
+    ] as const) {
+      const handler = vi.fn();
+      const onClose = vi.fn();
+      drill({ [key]: handler, onClose });
+      type(query);
+      press("Enter");
+      expect(handler).toHaveBeenCalledWith(expect.objectContaining({ id: "api" }));
+      expect(onClose).toHaveBeenCalled();
+      cleanup();
+    }
+  });
+
+  it("hands each action to the flow it already has, by default", () => {
+    for (const [query, event] of [
+      ["kill", KILL_SESSION_EVENT],
+      ["rename", RENAME_SESSION_EVENT],
+      ["spawn", SPAWN_SIMILAR_EVENT],
+    ] as const) {
+      const listener = vi.fn();
+      document.addEventListener(event, listener);
+      drill();
+      type(query);
+      press("Enter");
+      document.removeEventListener(event, listener);
+      expect((listener.mock.calls[0][0] as CustomEvent).detail.id).toBe("api");
+      cleanup();
+    }
+  });
+});

web/src/components/commandBar/CommandBar.tsx

@@ -0,0 +1,379 @@
+// The `ctrl-k` command bar (docs/COMMAND-BAR.md): a query line over a result
+// list. Sessions and commands rank together by default; `>` narrows to
+// commands, `~` to directories. `tab` on a session drills into its action page.
+//
+// The bar picks an action and hands off — it never reimplements one. Each
+// handler defaults to the event the flow that already implements it listens
+// on, so the React bar and the legacy shell raise the same flows.
+
+import { useCallback, useEffect, useMemo, useRef, useState } from "react";
+import { createPortal } from "react-dom";
+
+import {
+  KILL_SESSION_EVENT,
+  RENAME_SESSION_EVENT,
+  SESSION_SELECT_EVENT,
+  SPAWN_SIMILAR_EVENT,
+} from "../../catalog.js";
+import { publishedCommands, type Command } from "../../commands.js";
+import { sessionName, type Session } from "../../types.js";
+import { ActionPage } from "./ActionPage.js";
+import { ResultList } from "./ResultList.js";
+import {
+  barGroups,
+  barRows,
+  commandBarCandidates,
+  dirEntryPaths,
+  directoryBarCandidates,
+  isActionCandidate,
+  listDirEntries as defaultListDirEntries,
+  loadBarDirectories,
+  parseBarQuery,
+  probeDir,
+  sessionActionCandidates,
+  sessionBarCandidates,
+  PLACEHOLDERS,
+  type BarCandidate,
+  type BarResult,
+  type DirectoryBarCandidate,
+  type SessionAction,
+} from "./model.js";
+
+/** Raised when Enter lands on a directory row and no handler was given. The
+ *  bar picks the directory; the shell owns the spawn, which is the same
+ *  fast-forward path `N` takes. */
+export const SPAWN_DIRECTORY_EVENT = "cmdbar-spawn-directory";
+
+export interface CommandBarProps {
+  /** Sessions to build candidates from, as the store has them right now. */
+  sessions: readonly Session[];
+  /** The daemon's home dir, for the directory field's `~/…` abbreviation. */
+  homeDir?: string;
+  /** The command registry to list under `>`. Defaults to every bound command,
+   * in registry order. */
+  commands?: readonly Command[];
+  /** The directory list behind `~`, fetched once on first use. */
+  loadDirectories?: () => Promise<string[]>;
+  /** One directory's subdirectories, for filling the list in as a path is
+   * typed. Defaults to `GET /api/dirs`. */
+  listDirEntries?: (dir: string) => Promise<string[]>;
+  onOpenSession?(sessionId: string): void;
+  onSpawnDirectory?(path: string): void;
+  onKillSession?(session: Session): void;
+  onRenameSession?(session: Session): void;
+  onSpawnSimilar?(session: Session): void;
+  /** Closing returns the keyboard to whatever held it when the bar opened. */
+  onClose(): void;
+}
+
+function raise(name: string, detail: unknown): void {
+  document.dispatchEvent(new CustomEvent(name, { detail }));
+}
+
+export function CommandBar(props: CommandBarProps) {
+  const {
+    sessions,
+    homeDir,
+    commands,
+    loadDirectories = loadBarDirectories,
+    listDirEntries = defaultListDirEntries,
+    onClose,
+  } = props;
+
+  const inputRef = useRef<HTMLInputElement>(null);
+  const panelRef = useRef<HTMLDivElement>(null);
+
+  const [raw, setRaw] = useState("");
+  const [highlighted, setHighlighted] = useState(0);
+  // The action page: `page` is the session being acted on, `stashed` the
+  // result-list query it was drilled into from — backing out restores it, so
+  // the round trip costs the operator nothing.
+  const [page, setPage] = useState<Session | null>(null);
+  const [stashed, setStashed] = useState("");
+
+  // `~`'s candidates: the recents-and-projects list, fetched once, plus
+  // whatever `/api/dirs` has filled in for the directory being typed.
+  const [directories, setDirectories] = useState<DirectoryBarCandidate[]>([]);
+  const [probed, setProbed] = useState<DirectoryBarCandidate[]>([]);
+  const requestedRef = useRef(false);
+  const listingsRef = useRef(new Map<string, string[]>());
+
+  // On the action page the query line is over the three actions, so there is
+  // no sigil to read: `>` and `~` are ordinary characters there.
+  const parsed = useMemo(() => (page ? { sigil: null, query: raw } : parseBarQuery(raw)), [page, raw]);
+
+  const sessionCandidates = useMemo(
+    () => sessionBarCandidates(sessions, homeDir),
+    [sessions, homeDir],
+  );
+  const commandCandidates = useMemo(
+    () => commandBarCandidates(commands ?? publishedCommands()),
+    [commands],
+  );
+
+  // Closing returns the keyboard to whatever held it when the bar opened: a
+  // round trip through the bar never silently moves focus.
+  useEffect(() => {
+    const previouslyFocused = document.activeElement as HTMLElement | null;
+    inputRef.current?.focus();
+    return () => {
+      if (previouslyFocused?.isConnected) previouslyFocused.focus();
+    };
+  }, []);
+
+  // The recents-and-projects list, the first time `~` is used. No directories
+  // is a shorter list, not an error state.
+  useEffect(() => {
+    if (parsed.sigil !== "~" || requestedRef.current) return;
+    requestedRef.current = true;
+    let live = true;
+    loadDirectories()
+      .then((paths) => {
+        if (live) setDirectories(directoryBarCandidates(paths, homeDir));
+      })
+      .catch(() => {});
+    return () => {
+      live = false;
+    };
+  }, [parsed.sigil, loadDirectories, homeDir]);
+
+  // Fill the list in from `GET /api/dirs` once the query looks like a path: the
+  // directory it names is listed and its children join the candidates, so `~`
+  // reaches a directory that was never spawned in.
+  const dir = parsed.sigil === "~" ? probeDir(parsed.query) : "";
+  useEffect(() => {
+    if (dir === "") {
+      setProbed([]);
+      return;
+    }
+    const cached = listingsRef.current.get(dir);
+    if (cached) {
+      setProbed(directoryBarCandidates(dirEntryPaths(dir, cached), homeDir));
+      return;
+    }
+    let live = true;
+    setProbed([]);
+    listDirEntries(dir)
+      .then((entries) => {
+        listingsRef.current.set(dir, entries);
+        if (live) setProbed(directoryBarCandidates(dirEntryPaths(dir, entries), homeDir));
+      })
+      .catch(() => {
+        /* an unreadable directory contributes nothing */
+      });
+    return () => {
+      live = false;
+    };
+  }, [dir, listDirEntries, homeDir]);
+
+  /** What the matcher sees for the query line as it stands. With no sigil the
+   *  list is mixed — sessions and commands together — except on an empty query,
+   *  where the bar is a session switcher. */
+  const candidates: BarCandidate[] = useMemo(() => {
+    if (page) return sessionActionCandidates();
+    switch (parsed.sigil) {
+      case ">":
+        return [...commandCandidates];
+      case "~": {
+        const seen = new Set<string>();
+        return [...directories, ...probed].filter((c) =>
+          seen.has(c.path) ? false : (seen.add(c.path), true),
+        );
+      }
+      default:
+        return parsed.query === ""
+          ? [...sessionCandidates]
+          : [...sessionCandidates, ...commandCandidates];
+    }
+  }, [page, parsed.sigil, parsed.query, sessionCandidates, commandCandidates, directories, probed]);
+
+  const groups = useMemo(
+    () => barGroups(barRows(parsed.query, candidates)),
+    [parsed.query, candidates],
+  );
+  // Grouping is a rendering of the ranking, so the flattened groups are the
+  // rows in the order they appear — which is the order the highlight walks and
+  // Enter acts on.
+  const rows: BarResult[] = useMemo(() => groups.flatMap((g) => g.results), [groups]);
+
+  // The highlight starts on the globally best row every time the list changes:
+  // an old index would point at a different result.
+  useEffect(() => {
+    setHighlighted(0);
+  }, [rows]);
+
+  /** The highlight does not wrap: it stops at the first and last row. */
+  const move = useCallback(
+    (delta: number) => {
+      setHighlighted((at) => Math.min(rows.length - 1, Math.max(0, at + delta)));
+    },
+    [rows.length],
+  );
+
+  const runAction = useCallback(
+    (action: SessionAction, session: Session) => {
+      onClose();
+      switch (action.id) {
+        case "kill":
+          (props.onKillSession ?? ((s: Session) => raise(KILL_SESSION_EVENT, s)))(session);
+          return;
+        case "rename":
+          (props.onRenameSession ?? ((s: Session) => raise(RENAME_SESSION_EVENT, s)))(session);
+          return;
+        case "spawn-similar":
+          (props.onSpawnSimilar ?? ((s: Session) => raise(SPAWN_SIMILAR_EVENT, s)))(session);
+          return;
+      }
+    },
+    [onClose, props.onKillSession, props.onRenameSession, props.onSpawnSimilar],
+  );
+
+  const activate = useCallback(
+    (index: number) => {
+      const result = rows[index];
+      if (!result) return;
+      const candidate = result.candidate;
+      if (isActionCandidate(candidate)) {
+        // The page cannot be up without a session to act on.
+        if (page) runAction(candidate.action, page);
+        return;
+      }
+      onClose();
+      switch (candidate.kind) {
+        case "session":
+          (props.onOpenSession ?? ((id: string) => raise(SESSION_SELECT_EVENT, id)))(
+            candidate.session.id,
+          );
+          return;
+        case "command":
+          candidate.command.run();
+          return;
+        case "directory":
+          (props.onSpawnDirectory ?? ((p: string) => raise(SPAWN_DIRECTORY_EVENT, p)))(
+            candidate.path,
+          );
+          return;
+      }
+    },
+    [rows, page, runAction, onClose, props.onOpenSession, props.onSpawnDirectory],
+  );
+
+  /** `tab`: drill into the highlighted session's action page. No effect on any
+   *  other kind, and none on the page itself, which has no session rows. */
+  const drillIn = useCallback(() => {
+    const candidate = rows[highlighted]?.candidate;
+    if (page || candidate?.kind !== "session") return;
+    setPage(candidate.session);
+    setStashed(raw);
+    setRaw("");
+  }, [rows, highlighted, page, raw]);
+
+  /** Escape, or backspace at an empty query: back to the result list with the
+   *  query the drill-in was made from. */
+  const leavePage = useCallback(() => {
+    setPage(null);
+    setRaw(stashed);
+    setStashed("");
+  }, [stashed]);
+
+  const onKeyDown = (e: React.KeyboardEvent<HTMLInputElement>) => {
+    switch (e.key) {
+      case "Escape":
+        e.preventDefault();
+        // On an action page Escape is one level up, not the way out.
+        if (page) leavePage();
+        else onClose();
+        return;
+      case "Tab":
+        e.preventDefault();
+        drillIn();
+        return;
+      case "Backspace":
+        // Ordinary editing, except at an empty query on the action page.
+        if (page && raw === "") {
+          e.preventDefault();
+          leavePage();
+        }
+        return;
+      case "Enter":
+        e.preventDefault();
+        activate(highlighted);
+        return;
+      case "ArrowDown":
+        e.preventDefault();
+        move(1);
+        return;
+      case "ArrowUp":
+        e.preventDefault();
+        move(-1);
+        return;
+      case "n":
+        if (e.ctrlKey) {
+          e.preventDefault();
+          move(1);
+        }
+        return;
+      case "p":
+        if (e.ctrlKey) {
+          e.preventDefault();
+          move(-1);
+        }
+        return;
+    }
+  };
+
+  const activeRowId = rows.length > 0 ? `cmdbar-row-${highlighted}` : undefined;
+
+  return createPortal(
+    <div
+      className="cmdbar-overlay fixed inset-0 z-modal flex items-start justify-center bg-bg/70 pt-[12vh] backdrop-blur-sm max-[600px]:items-end max-[600px]:pt-0"
+      role="dialog"
+      aria-modal="true"
+      aria-label="Command bar"
+      onMouseDown={(e) => {
+        // A press on the scrim closes; one inside the panel is left alone.
+        if (!panelRef.current?.contains(e.target as Node)) onClose();
+      }}
+    >
+      <div
+        ref={panelRef}
+        className="cmdbar-panel flex w-full max-w-[560px] flex-col overflow-hidden rounded-r-3 border border-border bg-surface shadow-2 max-[600px]:max-w-none max-[600px]:rounded-b-none"
+      >
+        <input
+          ref={inputRef}
+          type="text"
+          role="combobox"
+          aria-expanded="true"
+          aria-controls="cmdbar-results"
+          aria-label="Search sessions, commands and directories"
+          aria-activedescendant={activeRowId}
+          autoComplete="off"
+          spellCheck={false}
+          className="cmdbar-input w-full border-b border-border bg-transparent px-sp-3 py-sp-2 text-fs-15 text-ink outline-none"
+          placeholder={page ? `Act on ${sessionName(page)}…` : PLACEHOLDERS[parsed.sigil ?? "mixed"]}
+          value={raw}
+          onChange={(e) => setRaw(e.currentTarget.value)}
+          onKeyDown={onKeyDown}
+        />
+        {page ? (
+          <ActionPage
+            session={page}
+            groups={groups}
+            highlighted={highlighted}
+            onActivate={activate}
+          />
+        ) : (
+          <ResultList
+            groups={groups}
+            // While a sigil is in force there is one kind, so the header is
+            // dropped.
+            showHeaders={parsed.sigil === null}
+            highlighted={highlighted}
+            onActivate={activate}
+          />
+        )}
+      </div>
+    </div>,
+    document.body,
+  );
+}

web/src/components/commandBar/ResultList.tsx

@@ -0,0 +1,171 @@
+// The bar's result list: kind headers over rank-ordered rows
+// (docs/COMMAND-BAR.md, "Structure"). Headers are labels, never rows — the
+// highlight walks the rows alone, in the order they are rendered.
+
+import { useEffect, useRef } from "react";
+
+import { keyCaps } from "../../commands.js";
+import {
+  highlightSegments,
+  isActionCandidate,
+  metaText,
+  rowName,
+  type BarGroup,
+  type BarResult,
+} from "./model.js";
+
+export interface ResultListProps {
+  groups: BarGroup[];
+  /** Dropped while a sigil is in force or on the action page: one kind needs
+   * no header. */
+  showHeaders: boolean;
+  /** Index into the flattened rows, in the order they render. */
+  highlighted: number;
+  onActivate(index: number): void;
+}
+
+function Highlighted({ text, ranges }: { text: string; ranges: BarResult["ranges"] }) {
+  return (
+    <>
+      {highlightSegments(text, ranges).map((segment, i) =>
+        segment.hit ? (
+          <mark key={i} className="cmdbar-hit bg-transparent font-semibold text-accent">
+            {segment.text}
+          </mark>
+        ) : (
+          <span key={i}>{segment.text}</span>
+        ),
+      )}
+    </>
+  );
+}
+
+function Row({
+  result,
+  index,
+  active,
+  onActivate,
+}: {
+  result: BarResult;
+  index: number;
+  active: boolean;
+  onActivate(index: number): void;
+}) {
+  const ref = useRef<HTMLDivElement>(null);
+  const candidate = result.candidate;
+
+  useEffect(() => {
+    if (active) ref.current?.scrollIntoView({ block: "nearest" });
+  }, [active]);
+
+  const name = rowName(candidate);
+  const nameMatched =
+    result.field === "name" || result.field === "label" || result.field === "path";
+  const meta = metaText(result);
+  const dead = candidate.kind === "session" && candidate.session.status === "Dead";
+  const chord =
+    candidate.kind === "command" && !isActionCandidate(candidate)
+      ? candidate.command.keys[0]
+      : undefined;
+
+  return (
+    <div
+      ref={ref}
+      role="option"
+      id={`cmdbar-row-${index}`}
+      aria-selected={active}
+      data-index={index}
+      data-kind={candidate.kind}
+      data-session-id={candidate.kind === "session" ? candidate.session.id : undefined}
+      data-action-id={isActionCandidate(candidate) ? candidate.action.id : undefined}
+      data-command-id={
+        candidate.kind === "command" && !isActionCandidate(candidate)
+          ? candidate.command.id
+          : undefined
+      }
+      data-path={candidate.kind === "directory" ? candidate.path : undefined}
+      className={[
+        "cmdbar-row flex cursor-pointer items-baseline gap-sp-2 rounded-r-1 px-sp-3 py-sp-1 text-fs-13",
+        active ? "is-active bg-surface-2 text-ink" : "text-ink",
+        dead ? "is-dead opacity-60" : "",
+      ]
+        .filter(Boolean)
+        .join(" ")}
+      onClick={() => onActivate(index)}
+    >
+      <span className="cmdbar-row-name truncate">
+        {nameMatched ? <Highlighted text={name} ranges={result.ranges} /> : name}
+      </span>
+      {chord !== undefined ? (
+        <span className="cmdbar-row-keys ml-auto flex gap-1">
+          {keyCaps(chord).map((cap, i) => (
+            <kbd
+              key={i}
+              className="rounded-r-1 border border-border px-1 text-fs-11 text-muted"
+            >
+              {cap}
+            </kbd>
+          ))}
+        </span>
+      ) : meta ? (
+        <span className="cmdbar-row-meta ml-auto truncate text-fs-11 text-muted">
+          {result.field !== undefined && result.field !== "name" ? (
+            <Highlighted text={meta} ranges={result.ranges} />
+          ) : (
+            meta
+          )}
+        </span>
+      ) : null}
+    </div>
+  );
+}
+
+export function ResultList({ groups, showHeaders, highlighted, onActivate }: ResultListProps) {
+  if (groups.length === 0) {
+    return (
+      <div
+        id="cmdbar-results"
+        role="listbox"
+        aria-label="Results"
+        className="cmdbar-results max-h-[50vh] overflow-auto p-sp-1"
+      >
+        <div className="cmdbar-empty px-sp-3 py-sp-2 text-fs-12 text-muted">No matches</div>
+      </div>
+    );
+  }
+
+  let index = 0;
+  return (
+    <div
+      id="cmdbar-results"
+      role="listbox"
+      aria-label="Results"
+      className="cmdbar-results max-h-[50vh] overflow-auto p-sp-1"
+    >
+      {groups.map((group) => (
+        <div key={group.kind}>
+          {showHeaders ? (
+            <div
+              role="presentation"
+              className="cmdbar-group-title px-sp-3 pt-sp-2 pb-1 text-fs-11 uppercase tracking-wide text-muted"
+            >
+              {group.title}
+            </div>
+          ) : null}
+          {group.results.map((result) => {
+            const at = index++;
+            return (
+              <Row
+                key={at}
+                result={result}
+                index={at}
+                active={at === highlighted}
+                onActivate={onActivate}
+              />
+            );
+          })}
+        </div>
+      ))}
+    </div>
+  );
+}

web/src/components/commandBar/index.ts

@@ -0,0 +1,5 @@
+export { CommandBar, SPAWN_DIRECTORY_EVENT, type CommandBarProps } from "./CommandBar.js";
+export { ResultList, type ResultListProps } from "./ResultList.js";
+export { ActionPage, type ActionPageProps } from "./ActionPage.js";
+export { useCommandBar, type CommandBarControls, type UseCommandBarOptions } from "./useCommandBar.js";
+export * from "./model.js";

web/src/components/commandBar/model.test.ts

@@ -0,0 +1,197 @@
+import { describe, expect, it } from "vitest";
+
+import type { Command } from "../../commands.js";
+import type { Session } from "../../types.js";
+import {
+  barGroups,
+  barRows,
+  commandBarCandidates,
+  dedupeCommands,
+  directoryBarCandidates,
+  directoryCandidate,
+  highlightSegments,
+  orderSessionsForBar,
+  parseBarQuery,
+  sessionActionCandidates,
+  sessionBarCandidates,
+  sessionCandidate,
+} from "./model.js";
+
+function session(id: string, over: Partial<Session> = {}): Session {
+  return {
+    id,
+    cwd: "/home/me/dev/api",
+    command: "bash",
+    env: {},
+    metadata: { name: id },
+    status: "Running",
+    created_at: "2026-01-01T00:00:00Z",
+    ...over,
+  };
+}
+
+function command(over: Partial<Command> = {}): Command {
+  return {
+    id: "a",
+    label: "A command",
+    group: "Test",
+    keys: ["a"],
+    scope: "global",
+    run: () => {},
+    ...over,
+  };
+}
+
+describe("empty-query ordering", () => {
+  it("sorts most recently focused first", () => {
+    const older = session("older", { last_focused_at: "2026-01-01T10:00:00Z" });
+    const newer = session("newer", { last_focused_at: "2026-01-01T11:00:00Z" });
+    expect(orderSessionsForBar([older, newer]).map((s) => s.id)).toEqual(["newer", "older"]);
+  });
+
+  it("sorts a never-focused session after every focused one, newest created first", () => {
+    const focused = session("focused", { last_focused_at: "2026-01-01T09:00:00Z" });
+    const old = session("old", { created_at: "2026-01-01T00:00:00Z" });
+    const fresh = session("fresh", { created_at: "2026-01-02T00:00:00Z" });
+    expect(orderSessionsForBar([old, fresh, focused]).map((s) => s.id)).toEqual([
+      "focused",
+      "fresh",
+      "old",
+    ]);
+  });
+
+  it("sorts dead sessions after live ones regardless of recency", () => {
+    const dead = session("dead", { status: "Dead", last_focused_at: "2026-01-01T12:00:00Z" });
+    const live = session("live", { last_focused_at: "2026-01-01T10:00:00Z" });
+    expect(orderSessionsForBar([dead, live]).map((s) => s.id)).toEqual(["live", "dead"]);
+  });
+
+  it("treats an empty last_focused_at as never focused", () => {
+    const blank = session("blank", { last_focused_at: "" });
+    const focused = session("focused", { last_focused_at: "2026-01-01T10:00:00Z" });
+    expect(orderSessionsForBar([blank, focused]).map((s) => s.id)).toEqual(["focused", "blank"]);
+  });
+});
+
+describe("session candidates", () => {
+  it("carries the candidate table's fields, with the directory abbreviated", () => {
+    const s = session("api", {
+      metadata: { name: "api" },
+      current_cwd: "/home/me/dev/api",
+      command: "claude --resume",
+    });
+    expect(sessionCandidate(s, "/home/me").fields).toEqual({
+      name: "api",
+      directory: "~/dev/api",
+      command: "claude --resume",
+      tool: "claude",
+    });
+  });
+
+  it("leaves the tool out when the program has no mark, and groups a worktree by its repo", () => {
+    const s = session("edit", {
+      metadata: {},
+      command: "hx src/",
+      worktree: { path: "/home/me/dev/api/wt/x", base_repo: "/home/me/dev/api" },
+    });
+    const candidate = sessionCandidate(s, "/home/me");
+    expect(candidate.fields.tool).toBeUndefined();
+    expect(candidate.fields.directory).toBe("~/dev/api");
+    // No `metadata.name`: the sidebar's fallback is the command line.
+    expect(candidate.fields.name).toBe("hx src/");
+  });
+
+  it("builds candidates in the empty-query order", () => {
+    const a = session("a", { last_focused_at: "2026-01-01T10:00:00Z" });
+    const b = session("b", { last_focused_at: "2026-01-01T11:00:00Z" });
+    expect(sessionBarCandidates([a, b]).map((c) => c.session.id)).toEqual(["b", "a"]);
+  });
+});
+
+describe("rows and grouping", () => {
+  it("an empty query keeps every candidate in input order", () => {
+    const candidates = sessionBarCandidates([session("a"), session("b")]);
+    expect(barRows("", candidates).map((r) => r.score)).toEqual([0, 0]);
+  });
+
+  it("groups by kind without reordering the ranking", () => {
+    const rows = barRows("kill", [
+      ...sessionBarCandidates([session("killer-app", { metadata: { name: "killer-app" } })]),
+      ...commandBarCandidates([command({ label: "Kill selected session" })]),
+    ]);
+    const groups = barGroups(rows);
+    expect(groups.map((g) => g.title)).toEqual(["Sessions", "Commands"]);
+    expect(groups.flatMap((g) => g.results)).toEqual(rows);
+  });
+});
+
+describe("parsing the query line", () => {
+  it("reads a sigil only in first position", () => {
+    expect(parseBarQuery(">help")).toEqual({ sigil: ">", query: "help" });
+    expect(parseBarQuery("~dev")).toEqual({ sigil: "~", query: "dev" });
+  });
+
+  it("leaves a sigil character elsewhere in the query literal", () => {
+    expect(parseBarQuery("~dev/~scratch")).toEqual({ sigil: "~", query: "dev/~scratch" });
+    expect(parseBarQuery("a>b")).toEqual({ sigil: null, query: "a>b" });
+  });
+
+  it("returns to the mixed list when the sigil is backspaced away", () => {
+    expect(parseBarQuery("")).toEqual({ sigil: null, query: "" });
+  });
+});
+
+describe("command candidates", () => {
+  it("dedupes by action, not by chord", () => {
+    const run = () => {};
+    const cmds = [
+      command({ id: "one", label: "Do it", keys: ["j"], run }),
+      command({ id: "two", label: "Do it", keys: ["down"], run }),
+    ];
+    expect(commandBarCandidates(cmds)).toHaveLength(1);
+    expect(dedupeCommands(cmds)).toHaveLength(1);
+  });
+
+  it("matches on the label only, never on the chord", () => {
+    const candidates = commandBarCandidates([command({ label: "Show help", keys: ["ctrl-q"] })]);
+    expect(barRows("ctrlq", candidates)).toEqual([]);
+    expect(barRows("show", candidates)).toHaveLength(1);
+  });
+});
+
+describe("directory candidates", () => {
+  it("abbreviates the path it shows and keeps the one it spawns in", () => {
+    const candidate = directoryCandidate("/home/me/dev/api", "/home/me");
+    expect(candidate.fields.path).toBe("~/dev/api");
+    expect(candidate.path).toBe("/home/me/dev/api");
+  });
+
+  it("expands a typed `~` and dedupes by the directory spawned in", () => {
+    const candidates = directoryBarCandidates(
+      ["~/dev/api", "/home/me/dev/api", "", "/srv/x"],
+      "/home/me",
+    );
+    expect(candidates.map((c) => c.path)).toEqual(["/home/me/dev/api", "/srv/x"]);
+  });
+});
+
+describe("the action page's candidates", () => {
+  it("offers no action beyond the three", () => {
+    expect(sessionActionCandidates().map((c) => c.action.id)).toEqual([
+      "kill",
+      "rename",
+      "spawn-similar",
+    ]);
+  });
+});
+
+describe("highlight segments", () => {
+  it("splits the text on the matcher's ranges", () => {
+    expect(highlightSegments("web-search", [[0, 1], [4, 5]])).toEqual([
+      { text: "w", hit: true },
+      { text: "eb-", hit: false },
+      { text: "s", hit: true },
+      { text: "earch", hit: false },
+    ]);
+  });
+});

web/src/components/commandBar/model.ts

@@ -0,0 +1,366 @@
+// The command bar's pure model (docs/COMMAND-BAR.md), lifted from
+// ../../commandBar.ts for the React port (#247): what a candidate is, the
+// order an empty query lists them in, how a query line splits into a sigil and
+// a query, and how a ranked list groups for rendering.
+//
+// Nothing here touches the DOM or the network apart from `loadBarDirectories`,
+// which is the one place the `~` list comes from.
+
+import { listDir, projects as fetchProjects, recentDirs } from "../../api.js";
+import type { Command } from "../../commands.js";
+import { splitPath } from "../../paths.js";
+import { splitCommand } from "../../toolIcon.js";
+import { abbreviateCwd, sessionName, sessionTreePath, type Session } from "../../types.js";
+import {
+  matchAndRank,
+  type CandidateKind,
+  type CommandCandidate,
+  type DirectoryCandidate,
+  type MatchRange,
+  type MatchResult,
+  type SessionCandidate,
+} from "../../utils/matcher.js";
+
+/** A matcher candidate that remembers what it came from, so a result can be
+ *  acted on without a second lookup. The extra property is invisible to the
+ *  matcher, which only ever reads `kind` and `fields`. */
+export type SessionBarCandidate = SessionCandidate & { session: Session };
+export type CommandBarCandidate = CommandCandidate & { command: Command };
+/** `fields.path` is the abbreviated path the operator reads and the matcher
+ *  ranks; `path` is the one a spawn is made in. */
+export type DirectoryBarCandidate = DirectoryCandidate & { path: string };
+
+/** One row of a session's action page. It is a `command` to the matcher — a
+ *  single `label` field, ranked exactly like a registry command — and carries
+ *  the action it stands for instead of a registry entry. */
+export type ActionBarCandidate = CommandCandidate & { action: SessionAction };
+
+export type BarCandidate =
+  | SessionBarCandidate
+  | CommandBarCandidate
+  | ActionBarCandidate
+  | DirectoryBarCandidate;
+export type BarResult = MatchResult<BarCandidate>;
+
+/** One kind's slice of the ranked list, in rank order. */
+export interface BarGroup<C extends BarCandidate = BarCandidate> {
+  kind: CandidateKind;
+  title: string;
+  results: MatchResult<C>[];
+}
+
+const KIND_TITLES: Record<CandidateKind, string> = {
+  session: "Sessions",
+  command: "Commands",
+  directory: "Directories",
+};
+
+// ---- the sigils ----
+
+export type BarSigil = ">" | "~";
+
+export interface BarQuery {
+  /** The sigil in force, or null for the mixed list. */
+  sigil: BarSigil | null;
+  /** What the matcher sees: everything after a sigil, or the whole query. */
+  query: string;
+}
+
+export const PLACEHOLDERS: Record<"mixed" | BarSigil, string> = {
+  mixed: "Go to a session…",
+  ">": "Run a command…",
+  "~": "Spawn in a directory…",
+};
+
+/** Split a raw query line into its sigil and the query proper.
+ *
+ *  A sigil is only a sigil in first position: `>` or `~` anywhere else is a
+ *  literal query character, which is what a directory query with a second `~`
+ *  in it depends on. Backspacing past a sigil needs no special case either —
+ *  the first character goes and the query is a mixed one again. */
+export function parseBarQuery(raw: string): BarQuery {
+  const first = raw[0];
+  if (first === ">" || first === "~") return { sigil: first, query: raw.slice(1) };
+  return { sigil: null, query: raw };
+}
+
+// ---- the session action page ----
+
+/** The actions a session's action page offers, and no others: the bar picks
+ *  one and hands off to the flow that already implements it
+ *  (docs/COMMAND-BAR.md, "Drill-in"). */
+export type SessionActionId = "kill" | "rename" | "spawn-similar";
+
+export interface SessionAction {
+  id: SessionActionId;
+  label: string;
+}
+
+export const SESSION_ACTIONS: readonly SessionAction[] = [
+  { id: "kill", label: "Kill session" },
+  { id: "rename", label: "Rename session" },
+  { id: "spawn-similar", label: "Spawn similar" },
+];
+
+/** The action page's candidates, in the order above — which is the order an
+ *  empty query lists them in, since the matcher keeps input order. */
+export function sessionActionCandidates(): ActionBarCandidate[] {
+  return SESSION_ACTIONS.map((action) => ({
+    kind: "command",
+    action,
+    fields: { label: action.label },
+  }));
+}
+
+/** Narrow a `command` candidate to an action row. Both share the matcher's
+ *  `command` kind, so what separates them is what they carry. */
+export function isActionCandidate(candidate: BarCandidate): candidate is ActionBarCandidate {
+  return candidate.kind === "command" && "action" in candidate;
+}
+
+// ---- candidates ----
+
+function focusedAt(s: Session): number | null {
+  const stamp = s.last_focused_at;
+  if (typeof stamp !== "string" || stamp === "") return null;
+  const ms = Date.parse(stamp);
+  return Number.isNaN(ms) ? null : ms;
+}
+
+function createdAt(s: Session): number {
+  const ms = Date.parse(s.created_at ?? "");
+  return Number.isNaN(ms) ? 0 : ms;
+}
+
+/** The empty-query order (docs/COMMAND-BAR.md, "Empty query"): most recently
+ *  focused first; a session that has never been focused sorts after every
+ *  session that has, most recently created first among those; and a dead
+ *  session sorts after every live one regardless of recency, because a session
+ *  you cannot attach to is not where you just were.
+ *
+ *  This is also the input order for a non-empty query, which is what the
+ *  matcher's last tie-break preserves. */
+export function orderSessionsForBar(sessions: readonly Session[]): Session[] {
+  return [...sessions].sort((a, b) => {
+    const deadA = a.status === "Dead" ? 1 : 0;
+    const deadB = b.status === "Dead" ? 1 : 0;
+    if (deadA !== deadB) return deadA - deadB;
+
+    const focusA = focusedAt(a);
+    const focusB = focusedAt(b);
+    if ((focusA === null) !== (focusB === null)) return focusA === null ? 1 : -1;
+    if (focusA !== null && focusB !== null && focusA !== focusB) return focusB - focusA;
+
+    return createdAt(b) - createdAt(a);
+  });
+}
+
+/** Build one session's candidate: the fields docs/COMMAND-BAR.md's candidate
+ *  table names, with the directory abbreviated the way the sidebar shows it. */
+export function sessionCandidate(s: Session, homeDir?: string): SessionBarCandidate {
+  const treePath = sessionTreePath(s);
+  const directory = treePath ? abbreviateCwd(treePath, homeDir) : undefined;
+  const tool = splitCommand(s.command).tool ?? undefined;
+  return {
+    kind: "session",
+    session: s,
+    fields: {
+      name: sessionName(s),
+      ...(directory ? { directory } : {}),
+      ...(s.command ? { command: s.command } : {}),
+      ...(tool ? { tool } : {}),
+    },
+  };
+}
+
+/** Every session as a candidate, in the empty-query order. */
+export function sessionBarCandidates(
+  sessions: readonly Session[],
+  homeDir?: string,
+): SessionBarCandidate[] {
+  return orderSessionsForBar(sessions).map((s) => sessionCandidate(s, homeDir));
+}
+
+/** Drop the commands that would list twice. Several chords can be bound to one
+ *  action — the registry already carries its aliases on one entry, but nothing
+ *  stops two entries running the same handler, and listed naively that is two
+ *  identical rows. The dedupe is by **action**, never by chord: the chord is
+ *  displayed, so a command with two of them is still one row. */
+export function dedupeCommands(commands: readonly Command[]): Command[] {
+  const seenRun = new Set<Command["run"]>();
+  const seenLabel = new Set<string>();
+  const kept: Command[] = [];
+  for (const command of commands) {
+    if (seenRun.has(command.run) || seenLabel.has(command.label)) continue;
+    seenRun.add(command.run);
+    seenLabel.add(command.label);
+    kept.push(command);
+  }
+  return kept;
+}
+
+/** Every bound command as a candidate, in registry order. Only the label is a
+ *  field: the chord is displayed beside the row, never matched, because
+ *  matching it would make `>` results depend on a user's rebinds. */
+export function commandBarCandidates(commands: readonly Command[]): CommandBarCandidate[] {
+  return dedupeCommands(commands).map((command) => ({
+    kind: "command",
+    command,
+    fields: { label: command.label },
+  }));
+}
+
+/** `~/dev/api` back to `/home/me/dev/api` — the inverse of `abbreviateCwd`, for
+ *  a path the operator typed or `/api/dirs` completed. */
+export function expandHome(path: string, homeDir?: string): string {
+  if (!homeDir) return path;
+  if (path === "~") return homeDir;
+  return path.startsWith("~/") ? `${homeDir}${path.slice(1)}` : path;
+}
+
+export function directoryCandidate(path: string, homeDir?: string): DirectoryBarCandidate {
+  const absolute = expandHome(path, homeDir);
+  return { kind: "directory", path: absolute, fields: { path: abbreviateCwd(absolute, homeDir) } };
+}
+
+/** Candidates for `~`, deduped by the directory they spawn in and keeping the
+ *  first mention of each — recent dirs (newest first) ahead of the project
+ *  list, which is the empty-query order the contract asks for. */
+export function directoryBarCandidates(
+  paths: readonly string[],
+  homeDir?: string,
+): DirectoryBarCandidate[] {
+  const seen = new Set<string>();
+  const out: DirectoryBarCandidate[] = [];
+  for (const path of paths) {
+    if (path === "") continue;
+    const candidate = directoryCandidate(path, homeDir);
+    if (seen.has(candidate.path)) continue;
+    seen.add(candidate.path);
+    out.push(candidate);
+  }
+  return out;
+}
+
+/** The directory list the bar starts from: what the spawn form already knows —
+ *  `GET /api/recent-dirs` and the project list — either alone if the other
+ *  fails. */
+export async function loadBarDirectories(): Promise<string[]> {
+  const [recents, discovered] = await Promise.all([
+    recentDirs()
+      .then((r) => r.dirs.map((d) => d.cwd))
+      .catch(() => [] as string[]),
+    fetchProjects()
+      .then((list) => list.map((p) => p.path))
+      .catch(() => [] as string[]),
+  ]);
+  return [...recents, ...discovered];
+}
+
+/** One directory's children as candidate paths, for filling the `~` list in as
+ *  a path is typed. The directory being listed comes from the query itself. */
+export function dirEntryPaths(dir: string, entries: readonly string[]): string[] {
+  return entries.map((entry) => `${dir}${entry}`);
+}
+
+/** The directory a `~` query names, or `""` when it names none yet. */
+export function probeDir(query: string): string {
+  return splitPath(query).dir;
+}
+
+/** `GET /api/dirs`, in the shape the bar wants it. */
+export function listDirEntries(dir: string): Promise<string[]> {
+  return listDir(dir).then((listing) => listing.entries);
+}
+
+// ---- grouping ----
+
+/** Bucket a ranked list by kind for rendering. Grouping is a rendering of the
+ *  ranking, never a reordering of it: a group holds its matches in rank order,
+ *  and groups appear in the order their best-scoring member does — so the first
+ *  row under the first header is the globally best result. */
+export function barGroups<C extends BarCandidate>(
+  results: readonly MatchResult<C>[],
+): BarGroup<C>[] {
+  const groups: BarGroup<C>[] = [];
+  for (const result of results) {
+    const kind = result.candidate.kind;
+    let group = groups.find((g) => g.kind === kind);
+    if (!group) {
+      group = { kind, title: KIND_TITLES[kind], results: [] };
+      groups.push(group);
+    }
+    group.results.push(result);
+  }
+  return groups;
+}
+
+/** The rows a query produces, flat and in rank order — what the highlight walks
+ *  and what Enter acts on. Headers are labels, not rows, so they are never in
+ *  this list. */
+export function barRows<C extends BarCandidate>(
+  query: string,
+  candidates: readonly C[],
+): MatchResult<C>[] {
+  return matchAndRank(query, candidates);
+}
+
+// ---- row text ----
+
+/** The name line of a row: the session's name, the command's label, or the
+ *  directory's path. */
+export function rowName(candidate: BarCandidate): string {
+  switch (candidate.kind) {
+    case "session":
+      return candidate.fields.name;
+    case "command":
+      return candidate.fields.label;
+    case "directory":
+      return candidate.fields.path;
+  }
+}
+
+/** The secondary line under a session row: the field the match was found in
+ *  when that was not the name, so a session found through its directory says
+ *  so. Only sessions have one — a command shows keycaps instead, and a
+ *  directory's path is already the name. */
+export function metaText(result: BarResult): string | undefined {
+  if (result.candidate.kind !== "session") return undefined;
+  const fields = result.candidate.fields;
+  switch (result.field) {
+    case "directory":
+      return fields.directory;
+    case "command":
+      return fields.command;
+    case "tool":
+      return fields.tool;
+    default:
+      return fields.directory;
+  }
+}
+
+/** One run of a row's text, flagged when the matcher matched inside it. The
+ *  React rendering of `renderHighlighted`: ranges come from the matcher as
+ *  `[start, end)` character pairs over the field that won. */
+export interface HighlightSegment {
+  text: string;
+  hit: boolean;
+}
+
+export function highlightSegments(
+  text: string,
+  ranges: readonly MatchRange[],
+): HighlightSegment[] {
+  const out: HighlightSegment[] = [];
+  let cursor = 0;
+  for (const [start, end] of ranges) {
+    if (start > cursor) out.push({ text: text.slice(cursor, start), hit: false });
+    out.push({ text: text.slice(start, end), hit: true });
+    cursor = end;
+  }
+  if (cursor < text.length) out.push({ text: text.slice(cursor), hit: false });
+  return out;
+}
+
+export { sessionName };
+export type { CandidateKind, MatchRange, MatchResult };

web/src/components/commandBar/useCommandBar.test.tsx

@@ -0,0 +1,76 @@
+// @vitest-environment happy-dom
+import { act, cleanup, render } from "@testing-library/react";
+import { afterEach, describe, expect, it, vi } from "vitest";
+
+import { OPEN_COMMAND_BAR_EVENT } from "../../catalog.js";
+import { useCommandBar, type UseCommandBarOptions } from "./useCommandBar.js";
+
+let controls: ReturnType<typeof useCommandBar>;
+
+function Probe(opts: UseCommandBarOptions) {
+  controls = useCommandBar(opts);
+  return <div>{controls.open ? "open" : "closed"}</div>;
+}
+
+function ctrlK(target: EventTarget = window): void {
+  act(() => {
+    target.dispatchEvent(
+      new KeyboardEvent("keydown", { key: "k", ctrlKey: true, bubbles: true, cancelable: true }),
+    );
+  });
+}
+
+afterEach(cleanup);
+
+describe("useCommandBar", () => {
+  it("opens on ctrl-k, from the capture phase so an attached xterm never sees it", () => {
+    const view = render(<Probe />);
+    ctrlK();
+    expect(view.container.textContent).toBe("open");
+  });
+
+  it("stays deaf while the bar is already up — its own input owns the keys", () => {
+    const onOpen = vi.fn();
+    render(<Probe onOpen={onOpen} />);
+    ctrlK();
+    ctrlK();
+    expect(onOpen).toHaveBeenCalledTimes(1);
+  });
+
+  it("leaves a form field's own Ctrl-K alone", () => {
+    const field = document.createElement("input");
+    document.body.appendChild(field);
+    const view = render(<Probe />);
+    ctrlK(field);
+    expect(view.container.textContent).toBe("closed");
+    field.remove();
+  });
+
+  it("opens from the sidebar's search field, the touch entry point", () => {
+    const view = render(<Probe />);
+    act(() => {
+      document.dispatchEvent(new CustomEvent(OPEN_COMMAND_BAR_EVENT));
+    });
+    expect(view.container.textContent).toBe("open");
+  });
+
+  it("runs the caller's hook before opening, so a preview is discarded first", () => {
+    const onOpen = vi.fn();
+    render(<Probe onOpen={onOpen} />);
+    ctrlK();
+    expect(onOpen).toHaveBeenCalledTimes(1);
+  });
+
+  it("closes on demand", () => {
+    const view = render(<Probe />);
+    ctrlK();
+    act(() => controls.closeBar());
+    expect(view.container.textContent).toBe("closed");
+  });
+
+  it("binds nothing while disabled — another overlay owns the keyboard", () => {
+    const view = render(<Probe enabled={false} />);
+    ctrlK();
+    expect(view.container.textContent).toBe("closed");
+  });
+});

web/src/components/commandBar/useCommandBar.ts

@@ -0,0 +1,65 @@
+// Opening and closing the bar (docs/COMMAND-BAR.md, "Opening"). `ctrl-k` is a
+// registry command like any other, so the `?` overlay lists it without a
+// second list; the touch entry point — the sidebar's search field — raises
+// `OPEN_COMMAND_BAR_EVENT` and lands on the same open.
+
+import { useCallback, useEffect, useState } from "react";
+
+import { OPEN_COMMAND_BAR_EVENT } from "../../catalog.js";
+import { CommandSet, defineCommands, isBarTypingTarget } from "../../commands.js";
+
+export interface UseCommandBarOptions {
+  /** Run before the bar opens — the shell discards a preview here, since the
+   * bar is not previewed into (docs/PREVIEW.md). */
+  onOpen?(): void;
+  /** Off while another overlay owns the keyboard (the restart overlay, the
+   * help dialog). Defaults to on. */
+  enabled?: boolean;
+}
+
+export interface CommandBarControls {
+  open: boolean;
+  openBar(): void;
+  closeBar(): void;
+}
+
+export function useCommandBar(opts: UseCommandBarOptions = {}): CommandBarControls {
+  const { onOpen, enabled = true } = opts;
+  const [open, setOpen] = useState(false);
+
+  const openBar = useCallback(() => {
+    onOpen?.();
+    setOpen(true);
+  }, [onOpen]);
+
+  const closeBar = useCallback(() => setOpen(false), []);
+
+  useEffect(() => {
+    if (!enabled) return;
+    const keys = new CommandSet(defineCommands({ "bar.open": openBar }));
+
+    // Capture phase: an attached xterm swallows every keystroke it is given, so
+    // the bar's binding has to be seen ahead of the terminal.
+    const onKeydown = (e: KeyboardEvent) => {
+      // The bar owns the keyboard while it is up; its own input handles the keys.
+      if (open) return;
+      // The pane's hidden textarea is not a typing target for the bar: ctrl-k
+      // is seen ahead of xterm and opens the bar. A real form field — the spawn
+      // form's inputs — keeps its own Ctrl+K.
+      if (isBarTypingTarget(e.target)) return;
+      keys.handle(e);
+    };
+    const onTouchEntry = () => {
+      if (!open) openBar();
+    };
+
+    window.addEventListener("keydown", onKeydown, true);
+    document.addEventListener(OPEN_COMMAND_BAR_EVENT, onTouchEntry);
+    return () => {
+      window.removeEventListener("keydown", onKeydown, true);
+      document.removeEventListener(OPEN_COMMAND_BAR_EVENT, onTouchEntry);
+    };
+  }, [enabled, open, openBar]);
+
+  return { open, openBar, closeBar };
+}

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

@@ -1,5 +1,5 @@
 // @vitest-environment happy-dom
-import { cleanup, fireEvent, render, screen } from "@testing-library/react";
+import { act, cleanup, fireEvent, render, screen } from "@testing-library/react";
 import { afterEach, describe, expect, it, vi } from "vitest";
 
 import { leaves } from "../../layout.js";
@@ -93,4 +93,60 @@ describe("MultiView", () => {
     render(<MultiView store={s} />);
     expect(screen.getByTestId("terminal-s1")).toBeTruthy();
   });
+
+  describe("preview mode (docs/PREVIEW.md)", () => {
+    it("shows a session no pane holds in a throwaway tab, marked as a preview", () => {
+      const s = store();
+      s.getState().openSession("s1");
+      render(<MultiView store={s} sessions={new Map([["s1", session("s1")]])} />);
+
+      act(() => s.getState().previewSession("s9"));
+      // s1's own tab is still in the strip — the preview never touched it.
+      expect(screen.getByText("Tab 1 · 1")).toBeTruthy();
+      const pane = document.querySelector('[data-session-id="s9"]');
+      expect(pane).toBeTruthy();
+      expect(pane!.querySelector(".bg-preview-tint")).toBeTruthy();
+      expect(screen.getByText("preview")).toBeTruthy();
+    });
+
+    it("puts the layout back byte for byte once the preview ends", () => {
+      const s = store();
+      s.getState().openSession("s1");
+      render(<MultiView store={s} sessions={new Map([["s1", session("s1")]])} />);
+      const before = JSON.stringify(s.getState().layout);
+
+      act(() => s.getState().previewSession("s9"));
+      act(() => s.getState().previewSession(null));
+
+      expect(JSON.stringify(s.getState().layout)).toBe(before);
+      expect(screen.queryByText("preview")).toBeNull();
+    });
+
+    it("does not grab the keyboard while previewing", () => {
+      const s = store();
+      s.getState().openSession("s1");
+      render(<MultiView store={s} sessions={new Map([["s1", session("s1")]])} />);
+
+      act(() => s.getState().previewSession("s9"));
+      const pane = document.querySelector('[data-session-id="s9"]') as HTMLElement;
+      expect(pane.querySelector('[data-testid="terminal-s9"]')).toBeTruthy();
+      // TerminalPane's `focus` prop is verified by TerminalPane's own tests;
+      // here the preview must never end up as the active-tab's accent pane.
+      expect(pane.className).not.toContain("border-accent");
+    });
+
+    it("clicking into a previewed pane commits it: no more preview, nothing moves a second time", () => {
+      const s = store();
+      s.getState().openSession("s1");
+      render(<MultiView store={s} sessions={new Map([["s1", session("s1")]])} />);
+
+      act(() => s.getState().previewSession("s9"));
+      const pane = document.querySelector('[data-session-id="s9"]') as HTMLElement;
+      fireEvent.pointerDown(pane);
+
+      expect(s.getState().preview).toBeNull();
+      expect(screen.queryByText("preview")).toBeNull();
+      expect(leaves(s.getState().layout.tabs[1].root).map((l) => l.sessionId)).toEqual(["s9"]);
+    });
+  });
 });

web/src/components/layout/MultiView.tsx

@@ -1,5 +1,5 @@
-// React port of multiview.ts's tab + split tree (issue #249). Preview mode
+// React port of multiview.ts's tab + split tree (issue #249) plus its preview
-// (docs/PREVIEW.md) is issue #251; sidebar drag-to-dock and the mobile
+// mode (issue #251, docs/PREVIEW.md). 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.
 
@@ -8,6 +8,7 @@ import { useEffect, useRef } from "react";
 import { useStore } from "zustand";
 
 import { activeTab, type Dir, type Node } from "../../layout.js";
+import { usePreview } from "../../hooks/usePreview.js";
 import type { LayoutStore, LayoutStoreApi } from "../../stores/layoutStore.js";
 import type { Session, Uuid } from "../../types.js";
 import { PaneContainer } from "./PaneContainer.js";
@@ -26,6 +27,8 @@ export interface MultiViewProps {
 export function MultiView({ store, sessions, onDisconnect, emptyState }: MultiViewProps) {
   const layout = useStore(store, (s: LayoutStore) => s.layout);
   const tab = activeTab(layout);
+  const { previewedSessionId, commit: commitPreview } = usePreview({ store });
+  const isPreviewing = previewedSessionId !== null;
 
   const reconciledRef = useRef(false);
   useEffect(() => {
@@ -37,9 +40,17 @@ export function MultiView({ store, sessions, onDisconnect, emptyState }: MultiVi
   return (
     <div className="mv flex h-full min-h-0 flex-col">
       <TabBar store={store} />
-      <div className="mv-canvas min-h-0 flex-1">
+      <div className="mv-canvas relative min-h-0 flex-1">
         {tab.root ? (
-          <SplitNode node={tab.root} tab={tab} store={store} sessions={sessions} onDisconnect={onDisconnect} />
+          <SplitNode
+            node={tab.root}
+            tab={tab}
+            store={store}
+            sessions={sessions}
+            onDisconnect={onDisconnect}
+            previewedSessionId={previewedSessionId}
+            commitPreview={commitPreview}
+          />
         ) : (
           (emptyState ?? (
             <div className="mv-empty flex h-full items-center justify-center p-sp-4 text-fs-13 text-faint">
@@ -47,6 +58,14 @@ export function MultiView({ store, sessions, onDisconnect, emptyState }: MultiVi
             </div>
           ))
         )}
+        {isPreviewing && (
+          <>
+            <div className="mv-preview-ring pointer-events-none absolute inset-0 rounded-r-2 border border-dashed border-preview-ring" />
+            <div className="mv-preview-chip pointer-events-none absolute right-sp-2 top-sp-2 z-sticky rounded-r-pill bg-preview px-sp-2 py-[2px] text-fs-11 text-preview-ink">
+              preview
+            </div>
+          </>
+        )}
       </div>
     </div>
   );
@@ -57,6 +76,8 @@ interface TreeProps {
   store: LayoutStoreApi;
   sessions?: ReadonlyMap<Uuid, Session>;
   onDisconnect?: (sessionId: Uuid) => void;
+  previewedSessionId: Uuid | null;
+  commitPreview: (sessionId: Uuid) => void;
 }
 
 function SplitNode({ node, ...rest }: TreeProps & { node: Node }) {
@@ -127,21 +148,34 @@ function Pane({
   store,
   sessions,
   onDisconnect,
+  previewedSessionId,
+  commitPreview,
 }: TreeProps & { leaf: Extract<Node, { kind: "leaf" }>; isFocused: boolean }) {
   const session = leaf.sessionId ? sessions?.get(leaf.sessionId) : undefined;
+  const isPreview = leaf.sessionId !== null && leaf.sessionId === previewedSessionId;
   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")
+        (isFocused && !isPreview ? "border-accent" : "border-border-strong")
       }
       data-leaf-id={leaf.id}
       data-session-id={leaf.sessionId ?? undefined}
-      onPointerDownCapture={() => store.getState().focusLeaf(leaf.id)}
+      onPointerDownCapture={() => {
+        // Clicking into a previewed pane is the operator choosing it
+        // (docs/PREVIEW.md).
+        if (isPreview) commitPreview(leaf.sessionId!);
+        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">
+      <div
+        className={
+          "mv-pane-header flex items-center justify-between gap-sp-2 border-b border-border-strong px-sp-2 py-sp-1 text-fs-12 " +
+          (isPreview ? "bg-preview-tint text-preview" : "bg-surface")
+        }
+      >
         {leaf.sessionId ? (
           <PaneTitle sessionId={leaf.sessionId} session={session} />
         ) : (
@@ -189,7 +223,7 @@ function Pane({
           <PaneContainer
             sessionId={leaf.sessionId}
             session={session}
-            focus={isFocused}
+            focus={isFocused && previewedSessionId === null}
             onClosed={() => store.getState().closeSession(leaf.sessionId!)}
           />
         ) : (

web/src/components/layout/TabBar.tsx

@@ -13,6 +13,7 @@ export interface TabBarProps {
 
 export function TabBar({ store }: TabBarProps) {
   const layout = useStore(store, (s: LayoutStore) => s.layout);
+  const previewTempTabId = useStore(store, (s: LayoutStore) => s.preview?.tempTabId ?? null);
   const [draggingId, setDraggingId] = useState<string | null>(null);
   const [overId, setOverId] = useState<string | null>(null);
 
@@ -22,6 +23,9 @@ export function TabBar({ store }: TabBarProps) {
         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;
+        // A preview's throwaway tab is not a place to go — it goes on its
+        // own the moment the preview ends (docs/PREVIEW.md).
+        const isPreviewTab = tab.id === previewTempTabId;
         return (
           <button
             key={tab.id}
@@ -32,7 +36,11 @@ export function TabBar({ store }: TabBarProps) {
             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") +
+              (isPreviewTab
+                ? "border border-dashed border-preview-ring italic text-preview"
+                : isActive
+                  ? "bg-surface-2 text-ink"
+                  : "text-faint hover:text-ink") +
               (isDropTarget ? " outline outline-2 outline-accent" : "")
             }
             onDragStart={(e) => {
@@ -57,13 +65,16 @@ export function TabBar({ store }: TabBarProps) {
               setDraggingId(null);
               setOverId(null);
             }}
-            onClick={() => store.getState().selectTab(tab.id)}
+            onClick={() => {
+              if (isPreviewTab) return;
+              store.getState().selectTab(tab.id);
+            }}
           >
             <span>
               {tab.title}
               {count ? ` · ${count}` : ""}
             </span>
-            {layout.tabs.length > 1 && (
+            {layout.tabs.length > 1 && !isPreviewTab && (
               <span
                 className="mv-tab-close text-faint hover:text-ink"
                 title="Close tab"

web/src/hooks/usePreview.test.tsx

@@ -0,0 +1,174 @@
+// @vitest-environment happy-dom
+import { act, cleanup, render } from "@testing-library/react";
+import { afterEach, describe, expect, it, vi } from "vitest";
+
+import { leaves } from "../layout.js";
+import { createLayoutStore, type LayoutStoreApi } from "../stores/layoutStore.js";
+import { usePreview, type UsePreviewResult } from "./usePreview.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();
+    },
+  };
+}
+
+/** Hand-driven clock + timer queue for the preview schedule's own throttle,
+ *  independent of the store's save-debounce scheduler above. */
+function fakeClock() {
+  let now = 1_000;
+  const timers = new Map<number, { at: number; fn: () => void }>();
+  let handle = 1;
+  return {
+    now: () => now,
+    setTimer: (fn: () => void, ms: number) => {
+      const h = handle++;
+      timers.set(h, { at: now + ms, fn });
+      return h;
+    },
+    clearTimer: (h: number) => {
+      timers.delete(h);
+    },
+    advance(ms: number) {
+      now += ms;
+      for (const [h, t] of [...timers]) {
+        if (t.at <= now) {
+          timers.delete(h);
+          act(() => t.fn());
+        }
+      }
+    },
+  };
+}
+
+function newStore(): LayoutStoreApi {
+  return createLayoutStore({ load: () => null, save: vi.fn(), scheduler: fakeScheduler().scheduler });
+}
+
+let controls: UsePreviewResult;
+
+function Probe({ store, clock }: { store: LayoutStoreApi; clock: ReturnType<typeof fakeClock> }) {
+  controls = usePreview({
+    store,
+    scheduleDeps: { now: clock.now, setTimer: clock.setTimer, clearTimer: clock.clearTimer },
+  });
+  return <div>{controls.previewedSessionId ?? "none"}</div>;
+}
+
+afterEach(cleanup);
+
+describe("usePreview", () => {
+  it("shows the first preview immediately — no settling delay", () => {
+    const store = newStore();
+    store.getState().openSession("s1");
+    const clock = fakeClock();
+    const view = render(<Probe store={store} clock={clock} />);
+
+    act(() => controls.preview("s9"));
+    expect(view.container.textContent).toBe("s9");
+    expect(leaves(store.getState().layout.tabs[1]?.root ?? null).map((l) => l.sessionId)).toEqual(["s9"]);
+  });
+
+  it("throttles a burst to one leading + one trailing pane swap", () => {
+    const store = newStore();
+    store.getState().openSession("s1");
+    const clock = fakeClock();
+    const view = render(<Probe store={store} clock={clock} />);
+
+    act(() => {
+      controls.preview("s1");
+      controls.preview("s8");
+      controls.preview("s9");
+    });
+    expect(view.container.textContent).toBe("s1");
+    clock.advance(120);
+    expect(view.container.textContent).toBe("s9");
+  });
+
+  it("touch never previews", () => {
+    const store = newStore();
+    store.getState().openSession("s1");
+    const clock = fakeClock();
+    const view = render(<Probe store={store} clock={clock} />);
+
+    act(() => controls.previewFromPointer("s9", "touch"));
+    expect(view.container.textContent).toBe("none");
+    expect(store.getState().preview).toBeNull();
+  });
+
+  it("previewFromPointer still previews for mouse/pen", () => {
+    const store = newStore();
+    store.getState().openSession("s1");
+    const clock = fakeClock();
+    const view = render(<Probe store={store} clock={clock} />);
+
+    act(() => controls.previewFromPointer("s9", "mouse"));
+    expect(view.container.textContent).toBe("s9");
+  });
+
+  it("commit cancels a pending trailing preview and opens for real", () => {
+    const store = newStore();
+    store.getState().openSession("s1");
+    const clock = fakeClock();
+    const view = render(<Probe store={store} clock={clock} />);
+
+    act(() => {
+      controls.preview("s1"); // leading edge: previews the already-open s1
+      controls.preview("s9"); // queued, gap not elapsed yet
+      controls.commit("s9");
+    });
+    expect(view.container.textContent).toBe("none"); // committed: no longer a preview
+    expect(leaves(store.getState().layout.tabs[0].root).map((l) => l.sessionId)).toEqual(["s9"]);
+
+    clock.advance(120);
+    // The cancelled trailing preview never fires.
+    expect(view.container.textContent).toBe("none");
+  });
+
+  it("end drops a preview immediately, bypassing the throttle", () => {
+    const store = newStore();
+    store.getState().openSession("s1");
+    const clock = fakeClock();
+    const view = render(<Probe store={store} clock={clock} />);
+
+    act(() => controls.preview("s9"));
+    expect(view.container.textContent).toBe("s9");
+
+    act(() => controls.end());
+    expect(view.container.textContent).toBe("none");
+    expect(store.getState().layout.tabs).toHaveLength(1);
+  });
+
+  it("cancels its pending trailing preview on unmount", () => {
+    const store = newStore();
+    store.getState().openSession("s1");
+    store.getState().addTab();
+    store.getState().openSession("s2"); // leading edge below will preview this, already on screen
+    const clock = fakeClock();
+    const view = render(<Probe store={store} clock={clock} />);
+
+    act(() => {
+      controls.preview("s2"); // leading edge: fires now
+      controls.preview("s9"); // queued trailing preview
+    });
+    expect(store.getState().preview?.sessionId).toBe("s2");
+    view.unmount();
+    clock.advance(120);
+    // The queued trailing preview must not fire after unmount.
+    expect(store.getState().preview?.sessionId).toBe("s2");
+  });
+});

web/src/hooks/usePreview.ts

@@ -0,0 +1,81 @@
+// React port of previewSchedule.ts + multiview.ts's preview controller
+// (issue #251). Owns the throttled schedule, hands the layout store the
+// throttled result, and exposes the three ways a preview ends for real —
+// see docs/PREVIEW.md, the contract this mirrors.
+
+import { useCallback, useEffect, useRef } from "react";
+import { useStore } from "zustand";
+
+import { createPreviewSchedule, type PreviewScheduleDeps } from "../previewSchedule.js";
+import type { LayoutStore, LayoutStoreApi } from "../stores/layoutStore.js";
+import type { Uuid } from "../types.js";
+
+export interface UsePreviewOptions {
+  store: LayoutStoreApi;
+  /** Test seam for the schedule's clock/timers. */
+  scheduleDeps?: PreviewScheduleDeps;
+}
+
+export interface UsePreviewResult {
+  /** The session currently shown as a preview, or null. */
+  previewedSessionId: Uuid | null;
+  /** Throttled: show `sessionId` in the content pane, or `null` to put back
+   *  whatever was on screen (a highlight move, or the highlight leaving
+   *  every session). Never delayed on the leading edge. */
+  preview(sessionId: Uuid | null): void;
+  /** Pointer-hover entry point: touch has no hover, so a touch pointer never
+   *  previews (docs/PREVIEW.md). */
+  previewFromPointer(sessionId: Uuid | null, pointerType: string): void;
+  /** Choosing the previewed session — click/Enter on its row, click into its
+   *  pane, or handing it the keyboard (Tab). Keeps exactly what is on
+   *  screen: a throwaway tab becomes real, a focus move inside an existing
+   *  tab stands. Nothing moves a second time. */
+  commit(sessionId: Uuid): void;
+  /** Any other real change — opening the command bar, a dialog, navigating
+   *  away — ends a pending or active preview immediately, bypassing the
+   *  throttle. */
+  end(): void;
+}
+
+/** Preview state management (docs/PREVIEW.md): wraps the layout store's
+ *  `previewSession`/`commitPreview` with the 120ms pane-swap throttle so the
+ *  UI never has to schedule it by hand. */
+export function usePreview({ store, scheduleDeps }: UsePreviewOptions): UsePreviewResult {
+  const previewedSessionId = useStore(store, (s: LayoutStore) => s.preview?.sessionId ?? null);
+
+  const scheduleRef = useRef<ReturnType<typeof createPreviewSchedule> | null>(null);
+  if (!scheduleRef.current) {
+    scheduleRef.current = createPreviewSchedule((id) => store.getState().previewSession(id), scheduleDeps);
+  }
+  useEffect(() => {
+    const schedule = scheduleRef.current;
+    return () => schedule?.cancel();
+  }, []);
+
+  const preview = useCallback((sessionId: Uuid | null) => {
+    scheduleRef.current?.(sessionId);
+  }, []);
+
+  const previewFromPointer = useCallback(
+    (sessionId: Uuid | null, pointerType: string) => {
+      if (pointerType === "touch") return;
+      preview(sessionId);
+    },
+    [preview],
+  );
+
+  const commit = useCallback(
+    (sessionId: Uuid) => {
+      scheduleRef.current?.cancel();
+      store.getState().openSession(sessionId);
+    },
+    [store],
+  );
+
+  const end = useCallback(() => {
+    scheduleRef.current?.cancel();
+    store.getState().previewSession(null);
+  }, [store]);
+
+  return { previewedSessionId, preview, previewFromPointer, commit, end };
+}

web/src/sidebarSearch.test.ts

@@ -7,7 +7,7 @@ import { beforeAll, describe, expect, it } from "vitest";
 
 import { OPEN_COMMAND_BAR_EVENT, renderSidebar } from "./catalog.js";
 import { sessionBarCandidates } from "./commandBar.js";
-import { matchAndRank } from "./matcher.js";
+import { matchAndRank } from "./utils/matcher.js";
 import type { CatalogListener, CatalogStore } from "./events.js";
 import type { Session } from "./types.js";
 

web/src/stores/layoutStore.test.ts

@@ -159,4 +159,133 @@ describe("layoutStore", () => {
     expect(save).toHaveBeenCalledTimes(1);
     expect(save.mock.calls[0][0]).toBe(s.getState().layout);
   });
+
+  describe("previewSession (docs/PREVIEW.md)", () => {
+    it("brings a session's own tab forward, then puts the layout back", () => {
+      const s = store();
+      s.getState().openSession("s1");
+      s.getState().addTab();
+      s.getState().openSession("s2");
+      const before = JSON.stringify(s.getState().layout);
+
+      s.getState().previewSession("s1");
+      const tab1 = s.getState().layout.tabs.find((t) => leaves(t.root).some((l) => l.sessionId === "s1"))!;
+      expect(s.getState().layout.activeTabId).toBe(tab1.id);
+      expect(s.getState().preview?.sessionId).toBe("s1");
+
+      s.getState().previewSession(null);
+      expect(s.getState().preview).toBeNull();
+      expect(JSON.stringify(s.getState().layout)).toBe(before);
+    });
+
+    it("shows a session no pane holds in a throwaway tab that leaves no trace", () => {
+      const s = store();
+      s.getState().openSession("s1");
+      const before = JSON.stringify(s.getState().layout);
+
+      s.getState().previewSession("s9");
+      expect(s.getState().layout.tabs).toHaveLength(2);
+      const tempTab = s.getState().layout.tabs[1];
+      expect(leaves(tempTab.root).map((l) => l.sessionId)).toEqual(["s9"]);
+
+      s.getState().previewSession(null);
+      expect(s.getState().layout.tabs).toHaveLength(1);
+      expect(JSON.stringify(s.getState().layout)).toBe(before);
+    });
+
+    it("stepping between previews leaves only the newest one on screen", () => {
+      const s = store();
+      s.getState().openSession("s1");
+      const before = JSON.stringify(s.getState().layout);
+
+      s.getState().previewSession("s9");
+      s.getState().previewSession("s8");
+      expect(s.getState().layout.tabs).toHaveLength(2);
+      expect(s.getState().preview?.sessionId).toBe("s8");
+
+      s.getState().previewSession(null);
+      expect(JSON.stringify(s.getState().layout)).toBe(before);
+    });
+
+    it("never persists while up, and saves again once settled back", () => {
+      const save = vi.fn();
+      const { scheduler, flush } = fakeScheduler();
+      const s = store({ save, scheduler });
+      s.getState().openSession("s1");
+      flush();
+      save.mockClear();
+
+      s.getState().previewSession("s9");
+      expect(save).not.toHaveBeenCalled();
+
+      s.getState().previewSession(null);
+      flush();
+      expect(save).toHaveBeenCalledTimes(1);
+    });
+
+    it("commitPreview keeps the throwaway tab as a real one", () => {
+      const s = store();
+      s.getState().openSession("s1");
+      s.getState().previewSession("s9");
+
+      expect(s.getState().commitPreview("s9")).toBe(true);
+      expect(s.getState().preview).toBeNull();
+      expect(s.getState().layout.tabs).toHaveLength(2);
+      expect(leaves(s.getState().layout.tabs[1].root).map((l) => l.sessionId)).toEqual(["s9"]);
+    });
+
+    it("commitPreview discards a non-matching preview and reports false", () => {
+      const s = store();
+      s.getState().openSession("s1");
+      s.getState().previewSession("s9");
+
+      expect(s.getState().commitPreview("s1")).toBe(false);
+      expect(s.getState().preview).toBeNull();
+      expect(s.getState().layout.tabs).toHaveLength(1);
+    });
+
+    it("openSession settles a matching preview instead of moving it a second time", () => {
+      const s = store();
+      s.getState().openSession("s1");
+      s.getState().previewSession("s9");
+
+      s.getState().openSession("s9");
+      expect(s.getState().preview).toBeNull();
+      expect(s.getState().layout.tabs).toHaveLength(2);
+      expect(leaves(s.getState().layout.tabs[0].root).map((l) => l.sessionId)).toEqual(["s1"]);
+    });
+
+    it("any real layout change ends a preview first: splitFocused", () => {
+      const s = store();
+      s.getState().openSession("s1");
+      s.getState().previewSession("s9");
+      const first = leaves(s.getState().layout.tabs[0].root)[0].id;
+
+      s.getState().splitFocused(first, "row");
+      expect(s.getState().preview).toBeNull();
+      expect(s.getState().layout.tabs).toHaveLength(1);
+      expect(leaves(s.getState().layout.tabs[0].root).map((l) => l.sessionId).sort()).toEqual([null, "s1"]);
+    });
+
+    it("any real layout change ends a preview first: addTab/selectTab/closeTab", () => {
+      const s = store();
+      s.getState().openSession("s1");
+      s.getState().previewSession("s9");
+      s.getState().addTab();
+      expect(s.getState().preview).toBeNull();
+      // The throwaway tab was discarded before the real tab was added.
+      expect(s.getState().layout.tabs).toHaveLength(2);
+    });
+
+    it("closeSession drops the preview before closing the real pane", () => {
+      const s = store();
+      s.getState().openSession("s1");
+      s.getState().previewSession("s9");
+
+      s.getState().closeSession("s1");
+      expect(s.getState().preview).toBeNull();
+      expect(s.getState().layout.tabs).toHaveLength(1);
+      expect(s.getState().layout.tabs[0].root).toBeNull();
+    });
+  });
 });

web/src/stores/layoutStore.ts

@@ -11,12 +11,14 @@ import {
   emptyLayout,
   equalizeRatios,
   findLeaf,
+  freshId,
   leaves,
   locateSession,
   removeLeaf,
   splitLeaf,
   type Dir,
   type Layout,
+  type Tab,
 } from "../layout.js";
 import { loadLayout, saveLayout } from "../persistence.js";
 import type { Uuid } from "../types.js";
@@ -35,13 +37,29 @@ export interface LayoutStoreOptions {
   scheduler?: Scheduler;
 }
 
+/** A provisional preview (docs/PREVIEW.md): records exactly what
+ *  `previewSession` moved, so ending it restores the layout byte-for-byte. */
+export interface PreviewState {
+  sessionId: Uuid;
+  /** Tab that was active before the preview. */
+  prevActiveTabId: string;
+  /** Focused leaf to restore, when the preview stole focus in a real tab. */
+  prevFocus: { tabId: string; leafId: string | null } | null;
+  /** Throwaway tab built for a session that no pane was showing. */
+  tempTabId: string | null;
+}
+
 export interface LayoutState {
   layout: Layout;
+  /** Session shown provisionally in the content pane, or null. A preview is
+   *  a view, not state: it never reaches `save` (docs/PREVIEW.md). */
+  preview: PreviewState | null;
 }
 
 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. */
+   *  focused pane (VS-Code-style replace), or seed an empty tab. Settles a
+   *  matching preview in place rather than moving it a second time. */
   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. */
@@ -66,6 +84,15 @@ export interface LayoutActions {
    *  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;
+  /** Show `sessionId` in the content pane without committing to it
+   *  (docs/PREVIEW.md): its own tab comes forward, or a throwaway tab
+   *  carries it. `null` puts back whatever was on screen before. Never
+   *  schedules a save while a preview is up. */
+  previewSession(sessionId: Uuid | null): void;
+  /** Keep what is being previewed: its throwaway tab becomes a real one, and
+   *  the focus it moved stands. Any other pending preview is simply
+   *  discarded. Returns whether `sessionId` was the one being previewed. */
+  commitPreview(sessionId: Uuid): boolean;
 }
 
 export type LayoutStore = LayoutState & LayoutActions;
@@ -107,10 +134,51 @@ export function createLayoutStore(opts: LayoutStoreOptions = {}) {
       commit();
     }
 
+    // ---- preview (docs/PREVIEW.md): a view, not a layout change — never
+    // schedules a save while it is up. ----
+
+    /** Undo the preview's moves. Mutates only — the caller re-renders /
+     *  persists as it sees fit. Returns whether a preview was active. */
+    function restorePreview(): boolean {
+      const p = get().preview;
+      if (!p) return false;
+      set({ preview: null });
+      const layout = get().layout;
+      if (p.tempTabId) {
+        const i = layout.tabs.findIndex((t) => t.id === p.tempTabId);
+        if (i >= 0) layout.tabs.splice(i, 1);
+      } else if (p.prevFocus) {
+        const t = layout.tabs.find((x) => x.id === p.prevFocus!.tabId);
+        if (t) t.focusedLeafId = p.prevFocus.leafId;
+      }
+      layout.activeTabId = layout.tabs.some((t) => t.id === p.prevActiveTabId)
+        ? p.prevActiveTabId
+        : layout.tabs[0].id;
+      return true;
+    }
+
+    function settlePreview(sessionId: Uuid): boolean {
+      const p = get().preview;
+      if (!p) return false;
+      if (p.sessionId !== sessionId) {
+        restorePreview();
+        return false;
+      }
+      set({ preview: null });
+      if (p.tempTabId) {
+        const layout = get().layout;
+        const t = layout.tabs.find((x) => x.id === p.tempTabId);
+        if (t) t.title = `Tab ${layout.tabs.length}`;
+      }
+      return true;
+    }
+
     return {
       layout: load() ?? emptyLayout(),
+      preview: null,
 
       openSession(sessionId) {
+        if (get().commitPreview(sessionId)) return;
         mutate((layout) => {
           const loc = locateSession(layout, sessionId);
           if (loc) {
@@ -130,6 +198,7 @@ export function createLayoutStore(opts: LayoutStoreOptions = {}) {
       },
 
       openSessionBeside(sessionId, placement) {
+        restorePreview();
         mutate((layout) => {
           const loc = locateSession(layout, sessionId);
           if (loc) {
@@ -158,6 +227,7 @@ export function createLayoutStore(opts: LayoutStoreOptions = {}) {
       },
 
       closeSession(sessionId) {
+        restorePreview();
         mutate((layout) => {
           const loc = locateSession(layout, sessionId);
           if (!loc) return;
@@ -170,6 +240,7 @@ export function createLayoutStore(opts: LayoutStoreOptions = {}) {
       },
 
       splitFocused(leafId, dir) {
+        restorePreview();
         mutate((layout) => {
           const tab = activeTab(layout);
           tab.focusedLeafId = splitLeaf(tab, leafId, null, dir, true);
@@ -178,6 +249,7 @@ export function createLayoutStore(opts: LayoutStoreOptions = {}) {
       },
 
       closePane(leafId) {
+        restorePreview();
         mutate((layout) => {
           const tab = activeTab(layout);
           removeLeaf(tab, leafId);
@@ -196,24 +268,28 @@ export function createLayoutStore(opts: LayoutStoreOptions = {}) {
       },
 
       selectTab(tabId) {
+        restorePreview();
         mutate((layout) => {
           if (layout.tabs.some((t) => t.id === tabId)) layout.activeTabId = tabId;
         });
       },
 
       addTab() {
+        restorePreview();
         mutate((layout) => {
           addTabToLayout(layout);
         });
       },
 
       closeTab(tabId) {
+        restorePreview();
         mutate((layout) => {
           closeTabInLayout(layout, tabId);
         });
       },
 
       reorderTab(fromId, toId) {
+        restorePreview();
         mutate((layout) => {
           if (fromId === toId) return;
           const from = layout.tabs.findIndex((t) => t.id === fromId);
@@ -248,6 +324,45 @@ export function createLayoutStore(opts: LayoutStoreOptions = {}) {
           for (const id of emptied) closeTabInLayout(layout, id);
         });
       },
+
+      previewSession(sessionId) {
+        const current = get().preview;
+        if (current?.sessionId === sessionId) return;
+        if (sessionId === null) {
+          if (restorePreview()) commit();
+          return;
+        }
+        restorePreview();
+        const layout = get().layout;
+        const prevActiveTabId = layout.activeTabId;
+        const loc = locateSession(layout, sessionId);
+        let preview: PreviewState;
+        if (loc) {
+          preview = {
+            sessionId,
+            prevActiveTabId,
+            prevFocus: { tabId: loc.tab.id, leafId: loc.tab.focusedLeafId },
+            tempTabId: null,
+          };
+          layout.activeTabId = loc.tab.id;
+          loc.tab.focusedLeafId = loc.leaf.id;
+        } else {
+          // Not on screen anywhere: a throwaway tab carries it, so no
+          // existing pane is evicted and nothing the operator arranged moves.
+          const tab: Tab = { id: freshId("t"), title: "Preview", root: null, focusedLeafId: null };
+          tab.focusedLeafId = splitLeaf(tab, "", sessionId, "row", true);
+          layout.tabs.push(tab);
+          layout.activeTabId = tab.id;
+          preview = { sessionId, prevActiveTabId, prevFocus: null, tempTabId: tab.id };
+        }
+        set({ layout: { ...layout }, preview });
+      },
+
+      commitPreview(sessionId) {
+        const ok = settlePreview(sessionId);
+        if (ok) commit();
+        return ok;
+      },
     };
   });
 }