feat(web): port daemonLink and restartOverlay to React (#242)

DaemonLink.tsx holds the link state machine as a Zustand store (same
shape as catalogStore/connectionStore) plus a thin controller component
wiring the catalog store's `connected` field to it. RestartOverlay.tsx
renders the blocking modal for down/updating, reusing the existing CSS.

Closes #242

web/src/components/daemon/DaemonLink.test.ts

@@ -0,0 +1,215 @@
+import { describe, expect, it, vi } from "vitest";
+
+import { GRACE_MS, POLL_MS, compareIdentity, createDaemonLinkStore, formatRelease, type Scheduler } from "./DaemonLink.js";
+import type { DaemonInfo } from "../../types.js";
+
+function info(commit: string, instance: string, date = "2026-09-02T14:03:11+01:00"): DaemonInfo {
+  return {
+    hostname: "konishi",
+    version: "0.1.0",
+    presets: [],
+    build: { commit, commit_date: date, dirty: false },
+    instance,
+  };
+}
+
+/** Scheduler whose timers only fire when the test says so, recording the
+ *  delay each was scheduled with. */
+function manual(): Scheduler & { run(): void; pending(): number; delays: number[] } {
+  let next = 1;
+  const timers = new Map<number, () => void>();
+  const delays: number[] = [];
+  return {
+    delays,
+    setTimer(fn, ms) {
+      const h = next++;
+      timers.set(h, fn);
+      delays.push(ms);
+      return h;
+    },
+    clearTimer(h) {
+      timers.delete(h);
+    },
+    run() {
+      const due = [...timers.entries()];
+      timers.clear();
+      for (const [, fn] of due) fn();
+    },
+    pending: () => timers.size,
+  };
+}
+
+describe("compareIdentity", () => {
+  it("reads a changed commit as an upgrade", () => {
+    expect(compareIdentity(info("aaa", "i1"), info("bbb", "i2"))).toBe("upgraded");
+  });
+
+  it("reads a changed instance on the same commit as a plain restart", () => {
+    expect(compareIdentity(info("aaa", "i1"), info("aaa", "i2"))).toBe("restarted");
+  });
+
+  it("reads an unchanged process as no event at all", () => {
+    expect(compareIdentity(info("aaa", "i1"), info("aaa", "i1"))).toBe("same");
+  });
+
+  it("falls back to the instance when the build is unknown", () => {
+    // A daemon built outside a checkout reports empty commits; the instance is
+    // still enough to see the restart, and never claims a phantom upgrade.
+    expect(compareIdentity(info("", "i1"), info("", "i2"))).toBe("restarted");
+    expect(compareIdentity(info("", "i1"), info("", "i1"))).toBe("same");
+  });
+});
+
+describe("formatRelease", () => {
+  it("pairs the short hash with a readable date", () => {
+    expect(formatRelease({ commit: "f89be4a716c7", commit_date: "2026-09-02T14:03:11Z", dirty: false })).toBe(
+      "f89be4a716c7 · 2026-09-02",
+    );
+  });
+
+  it("marks a dirty build", () => {
+    expect(formatRelease({ commit: "f89be4a", commit_date: "", dirty: true })).toBe("f89be4a (dirty)");
+  });
+
+  it("says so when the release is unknown", () => {
+    expect(formatRelease(undefined)).toBe("unknown release");
+  });
+});
+
+describe("daemon link store", () => {
+  it("starts in the connecting state until the first probe lands", async () => {
+    const store = createDaemonLinkStore({
+      fetchInfo: async () => info("aaa", "i1"),
+      scheduler: manual(),
+      reload: () => {},
+    });
+    expect(store.getState().state).toBe("connecting");
+    await store.getState().probe();
+    expect(store.getState().state).toBe("online");
+    expect(store.getState().build?.commit).toBe("aaa");
+  });
+
+  it("holds the modal back during the grace period, then blocks", async () => {
+    const sched = manual();
+    const store = createDaemonLinkStore({
+      fetchInfo: async () => {
+        throw new Error("down");
+      },
+      scheduler: sched,
+      reload: () => {},
+      graceMs: 1000,
+    });
+    store.getState().noteDisconnected();
+    // Still inside the grace window: the dot goes amber, nothing blocks.
+    expect(store.getState().state).toBe("reconnecting");
+    sched.run();
+    await Promise.resolve();
+    expect(store.getState().state).toBe("down");
+  });
+
+  it("unlocks without reloading when the same build comes back", async () => {
+    const sched = manual();
+    const reload = vi.fn();
+    let reply: DaemonInfo | null = info("aaa", "i1");
+    const store = createDaemonLinkStore({
+      fetchInfo: async () => {
+        if (!reply) throw new Error("down");
+        return reply;
+      },
+      scheduler: sched,
+      reload,
+    });
+    await store.getState().probe();
+
+    reply = null;
+    store.getState().noteDisconnected();
+    sched.run(); // grace expires → blocked, poll scheduled
+    await Promise.resolve();
+    expect(store.getState().state).toBe("down");
+
+    // Same commit, new process: a restart, not an upgrade — the page stays.
+    reply = info("aaa", "i2");
+    await store.getState().probe();
+    expect(reload).not.toHaveBeenCalled();
+    expect(store.getState().state).toBe("online");
+  });
+
+  it("reloads the page when the daemon comes back on a new commit", async () => {
+    const sched = manual();
+    const reload = vi.fn();
+    let reply = info("aaa", "i1");
+    const store = createDaemonLinkStore({
+      fetchInfo: async () => reply,
+      scheduler: sched,
+      reload,
+    });
+    await store.getState().probe();
+
+    store.getState().noteDisconnected();
+    reply = info("bbb", "i2");
+    await store.getState().probe();
+    expect(reload).toHaveBeenCalledTimes(1);
+    // Left blocked deliberately: the reload is what ends this page's life, and
+    // flashing an unlocked UI in between would let the user act on stale code.
+    expect(store.getState().state).toBe("updating");
+  });
+
+  it("keeps polling on a scheduled cadence while the daemon is unreachable", async () => {
+    const sched = manual();
+    let attempts = 0;
+    const store = createDaemonLinkStore({
+      fetchInfo: async () => {
+        attempts += 1;
+        throw new Error("down");
+      },
+      scheduler: sched,
+      reload: () => {},
+    });
+    store.getState().noteDisconnected();
+    sched.run(); // grace → first poll
+    await Promise.resolve();
+    await Promise.resolve();
+    const first = attempts;
+    expect(first).toBeGreaterThan(0);
+    sched.run(); // next poll tick
+    await Promise.resolve();
+    await Promise.resolve();
+    expect(attempts).toBeGreaterThan(first);
+    store.getState().stop();
+  });
+
+  it("notifies subscribers on every state change", async () => {
+    const sched = manual();
+    const seen: string[] = [];
+    const store = createDaemonLinkStore({
+      fetchInfo: async () => info("aaa", "i1"),
+      scheduler: sched,
+      reload: () => {},
+    });
+    store.subscribe((s) => seen.push(s.state));
+    await store.getState().probe();
+    store.getState().noteDisconnected();
+    sched.run();
+    await Promise.resolve();
+    expect(seen).toContain("reconnecting");
+  });
+
+  it("schedules the grace timer and poll at the documented defaults", async () => {
+    const sched = manual();
+    const store = createDaemonLinkStore({
+      fetchInfo: async () => {
+        throw new Error("down");
+      },
+      scheduler: sched,
+      reload: () => {},
+    });
+    expect(GRACE_MS).toBe(1200);
+    expect(POLL_MS).toBe(700);
+    store.getState().noteDisconnected();
+    expect(sched.delays).toEqual([GRACE_MS]);
+    sched.run(); // grace fires → schedules the first poll
+    await Promise.resolve();
+    await Promise.resolve();
+    expect(sched.delays).toEqual([GRACE_MS, POLL_MS]);
+  });
+});

web/src/components/daemon/DaemonLink.test.tsx

@@ -0,0 +1,71 @@
+// @vitest-environment happy-dom
+import { cleanup, render } from "@testing-library/react";
+import { afterEach, describe, expect, it, vi } from "vitest";
+
+import { createDaemonLinkStore } from "./DaemonLink.js";
+import { DaemonLink } from "./DaemonLink.js";
+
+afterEach(() => cleanup());
+
+describe("DaemonLink", () => {
+  it("probes once on mount, regardless of the initial connected value", async () => {
+    const fetchInfo = vi.fn().mockResolvedValue({ hostname: "h", version: "1", presets: [] });
+    const store = createDaemonLinkStore({ fetchInfo, scheduler: { setTimer: () => 0, clearTimer: () => {} } });
+    render(<DaemonLink store={store} connected={false} />);
+    await Promise.resolve();
+    expect(fetchInfo).toHaveBeenCalledTimes(1);
+  });
+
+  it("does not treat the initial connected value as a transition", async () => {
+    const store = createDaemonLinkStore({
+      fetchInfo: async () => ({ hostname: "h", version: "1", presets: [] }),
+      scheduler: { setTimer: () => 0, clearTimer: () => {} },
+    });
+    render(<DaemonLink store={store} connected={true} />);
+    await Promise.resolve();
+    // Only the mount probe ran — no extra noteConnected() probe on top of it.
+    expect(store.getState().state).toBe("online");
+  });
+
+  it("forwards a later connected drop to noteDisconnected", async () => {
+    const store = createDaemonLinkStore({
+      fetchInfo: async () => ({ hostname: "h", version: "1", presets: [] }),
+      scheduler: { setTimer: () => 0, clearTimer: () => {} },
+    });
+    const { rerender } = render(<DaemonLink store={store} connected={true} />);
+    await Promise.resolve();
+    expect(store.getState().state).toBe("online");
+
+    rerender(<DaemonLink store={store} connected={false} />);
+    expect(store.getState().state).toBe("reconnecting");
+  });
+
+  it("forwards a later connected recovery to noteConnected", async () => {
+    const fetchInfo = vi.fn().mockResolvedValue({ hostname: "h", version: "1", presets: [] });
+    const store = createDaemonLinkStore({ fetchInfo, scheduler: { setTimer: () => 0, clearTimer: () => {} } });
+    const { rerender } = render(<DaemonLink store={store} connected={true} />);
+    await Promise.resolve();
+    expect(store.getState().state).toBe("online");
+
+    // The socket dropped: noteDisconnected() moves off "online" so the coming
+    // recovery isn't a no-op.
+    rerender(<DaemonLink store={store} connected={false} />);
+    expect(store.getState().state).toBe("reconnecting");
+    fetchInfo.mockClear();
+
+    rerender(<DaemonLink store={store} connected={true} />);
+    await Promise.resolve();
+    expect(fetchInfo).toHaveBeenCalledTimes(1);
+    expect(store.getState().state).toBe("online");
+  });
+
+  it("stops the store's polling on unmount", () => {
+    const store = createDaemonLinkStore({
+      fetchInfo: async () => ({ hostname: "h", version: "1", presets: [] }),
+    });
+    const stop = vi.spyOn(store.getState(), "stop");
+    const { unmount } = render(<DaemonLink store={store} connected={false} />);
+    unmount();
+    expect(stop).toHaveBeenCalledTimes(1);
+  });
+});

web/src/components/daemon/DaemonLink.tsx

@@ -0,0 +1,234 @@
+// React port of ../../daemonLink.ts. The link to the daemon behind this page:
+// is it up, and which release is it? `RestartOverlay` (./RestartOverlay.tsx)
+// renders what this reports. See docs/RESTART.md for the restart contract.
+
+import { useEffect, useRef } from "react";
+import { create } from "zustand";
+
+import { getDaemonInfo } from "../../api.js";
+import type { BuildInfo, DaemonInfo } from "../../types.js";
+
+/**
+ * - `connecting` — first probe hasn't landed yet.
+ * - `online` — the daemon answered and is the same process, or a resolved new one.
+ * - `reconnecting` — the socket dropped, but within the grace window; a blip
+ *   shouldn't throw a modal in the user's face.
+ * - `down` — unreachable past the grace window: the overlay blocks the UI and
+ *   the poll runs until it answers.
+ * - `updating` — it answered on a different commit; the page is reloading and
+ *   stays blocked until it does.
+ */
+export type LinkState = "connecting" | "online" | "reconnecting" | "down" | "updating";
+
+/** What happened between two `/api/info` replies. */
+export type RestartKind = "same" | "restarted" | "upgraded";
+
+/** How long a dropped socket may stay dropped before the UI blocks. Long enough
+ *  to ride out a wifi hiccup, short enough that a real restart is visible. */
+export const GRACE_MS = 1200;
+/** Cadence of the `/api/info` poll while the daemon is unreachable. */
+export const POLL_MS = 700;
+
+/**
+ * Compare two daemon replies.
+ *
+ * The commit decides first: a different one means new code, which for a browser
+ * means new assets. The per-process `instance` catches the rest — a restart of
+ * the same build, which needs no reload. A daemon built outside a git checkout
+ * reports an empty commit, so an empty-vs-empty pair never counts as an
+ * upgrade; the instance still reports the restart.
+ */
+export function compareIdentity(before: DaemonInfo, after: DaemonInfo): RestartKind {
+  const a = before.build?.commit ?? "";
+  const b = after.build?.commit ?? "";
+  if (a && b && a !== b) return "upgraded";
+  if (before.instance && after.instance && before.instance !== after.instance) return "restarted";
+  return "same";
+}
+
+/** One line naming a release: `f89be4a716c7 · 2026-09-02`, with `(dirty)` when
+ *  the build carried uncommitted changes. Empty builds say so rather than
+ *  rendering a blank tooltip. */
+export function formatRelease(build: BuildInfo | undefined): string {
+  if (!build?.commit) return "unknown release";
+  const day = build.commit_date ? build.commit_date.slice(0, 10) : "";
+  const parts = [build.commit];
+  if (day) parts.push(`· ${day}`);
+  if (build.dirty) parts.push("(dirty)");
+  return parts.join(" ");
+}
+
+/** Schedules a probe/grace timer; returns a handle `clearTimer` understands. */
+export interface Scheduler {
+  setTimer(fn: () => void, ms: number): number;
+  clearTimer(handle: number): void;
+}
+
+const browserScheduler: Scheduler = {
+  setTimer: (fn, ms) => window.setTimeout(fn, ms),
+  clearTimer: (h) => window.clearTimeout(h),
+};
+
+export interface DaemonLinkState {
+  state: LinkState;
+  /** The release the daemon last reported. Undefined until the first probe,
+   *  and on a daemon too old to carry one. */
+  build?: BuildInfo;
+  /** Hostname the daemon reports, for the indicator's tooltip. */
+  hostname?: string;
+  /** How many probes have failed in the current outage — the overlay counts
+   *  attempts so a long restart doesn't look frozen. */
+  attempts: number;
+}
+
+export interface DaemonLinkActions {
+  /** One `/api/info` round-trip. Resolves `true` when the daemon answered. */
+  probe(): Promise<boolean>;
+  /** The events socket dropped. Starts the grace timer and the poll. */
+  noteDisconnected(): void;
+  /** The events socket came back. It carries no identity of its own, so the
+   *  state only clears once a probe confirms *which* daemon answered — a
+   *  reconnect to an upgraded daemon must reload, not resume. */
+  noteConnected(): void;
+  /** Stop polling. */
+  stop(): void;
+}
+
+export type DaemonLinkStore = DaemonLinkState & DaemonLinkActions;
+
+export interface DaemonLinkOptions {
+  /** Injectable for tests; defaults to `GET /api/info`. */
+  fetchInfo?: () => Promise<DaemonInfo>;
+  scheduler?: Scheduler;
+  /** What an upgrade does. Injectable so tests don't navigate. */
+  reload?: () => void;
+  graceMs?: number;
+  pollMs?: number;
+}
+
+/** Builds an isolated daemon-link store — same shape as
+ *  {@link ../../stores/catalogStore.js!createCatalogStore}: both a React hook
+ *  and a plain object (`getState`/`subscribe`), so it drives the overlay
+ *  whether or not React is the one calling `noteConnected`/`noteDisconnected`. */
+export function createDaemonLinkStore(opts: DaemonLinkOptions = {}) {
+  const fetchInfo = opts.fetchInfo ?? getDaemonInfo;
+  const scheduler = opts.scheduler ?? browserScheduler;
+  const reload = opts.reload ?? (() => window.location.reload());
+  const graceMs = opts.graceMs ?? GRACE_MS;
+  const pollMs = opts.pollMs ?? POLL_MS;
+
+  let identity: DaemonInfo | null = null;
+  let graceTimer: number | null = null;
+  let pollTimer: number | null = null;
+
+  return create<DaemonLinkStore>((set, get) => {
+    function clearTimers(): void {
+      if (graceTimer !== null) {
+        scheduler.clearTimer(graceTimer);
+        graceTimer = null;
+      }
+      if (pollTimer !== null) {
+        scheduler.clearTimer(pollTimer);
+        pollTimer = null;
+      }
+    }
+
+    async function probe(): Promise<boolean> {
+      let info: DaemonInfo;
+      try {
+        info = await fetchInfo();
+      } catch {
+        set((s) => ({ attempts: s.attempts + 1 }));
+        return false;
+      }
+      const before = identity;
+      identity = info;
+      if (before && compareIdentity(before, info) === "upgraded") {
+        // Stay blocked through the reload: an unlocked frame here would let the
+        // user drive new server code from the bundle it just replaced.
+        set({ state: "updating", build: info.build, hostname: info.hostname });
+        reload();
+        return true;
+      }
+      clearTimers();
+      set({ state: "online", build: info.build, hostname: info.hostname, attempts: 0 });
+      return true;
+    }
+
+    async function poll(): Promise<void> {
+      const state = get().state;
+      if (state === "online" || state === "updating") return;
+      const ok = await probe();
+      if (ok || pollTimer !== null) return;
+      pollTimer = scheduler.setTimer(() => {
+        pollTimer = null;
+        void poll();
+      }, pollMs);
+    }
+
+    return {
+      state: "connecting",
+      build: undefined,
+      hostname: undefined,
+      attempts: 0,
+
+      probe,
+
+      noteDisconnected() {
+        const state = get().state;
+        if (state === "down" || state === "updating") return;
+        set({ state: "reconnecting", attempts: 0 });
+        if (graceTimer === null) {
+          graceTimer = scheduler.setTimer(() => {
+            graceTimer = null;
+            if (get().state === "reconnecting") set({ state: "down" });
+            void poll();
+          }, graceMs);
+        }
+      },
+
+      noteConnected() {
+        const state = get().state;
+        if (state === "online" || state === "updating") return;
+        void probe();
+      },
+
+      stop: clearTimers,
+    };
+  });
+}
+
+export type DaemonLinkStoreApi = ReturnType<typeof createDaemonLinkStore>;
+
+export interface DaemonLinkProps {
+  /** The link store this instance drives — `RestartOverlay` (and, later, the
+   *  sidebar's status indicator) read the same one. */
+  store: DaemonLinkStoreApi;
+  /** The catalog store's `connected` field (docs/RESTART.md): the events
+   *  socket dying is the earliest signal a restart happened, ahead of any
+   *  poll. Pass `useStore(catalogStore, (s) => s.connected)`. */
+  connected: boolean;
+}
+
+/**
+ * Renders nothing — the controller that wires a `DaemonLinkStore` to the
+ * catalog's socket state. Mount once near the app root; `RestartOverlay`
+ * reads the store it drives.
+ */
+export function DaemonLink({ store, connected }: DaemonLinkProps): null {
+  const mounted = useRef(false);
+
+  useEffect(() => {
+    if (!mounted.current) {
+      mounted.current = true;
+      void store.getState().probe();
+      return;
+    }
+    if (connected) store.getState().noteConnected();
+    else store.getState().noteDisconnected();
+  }, [store, connected]);
+
+  useEffect(() => () => store.getState().stop(), [store]);
+
+  return null;
+}

web/src/components/daemon/RestartOverlay.test.tsx

@@ -0,0 +1,78 @@
+// @vitest-environment happy-dom
+import { cleanup, fireEvent, render, screen } from "@testing-library/react";
+import { afterEach, describe, expect, it, vi } from "vitest";
+
+import { createDaemonLinkStore, type DaemonLinkStoreApi } from "./DaemonLink.js";
+import { RestartOverlay } from "./RestartOverlay.js";
+
+afterEach(() => cleanup());
+
+function storeAt(state: "connecting" | "online" | "reconnecting" | "down" | "updating"): DaemonLinkStoreApi {
+  const store = createDaemonLinkStore({ scheduler: { setTimer: () => 0, clearTimer: () => {} } });
+  store.setState({ state });
+  return store;
+}
+
+describe("RestartOverlay", () => {
+  it("renders nothing while connecting, online, or reconnecting", () => {
+    for (const state of ["connecting", "online", "reconnecting"] as const) {
+      const { unmount } = render(<RestartOverlay store={storeAt(state)} />);
+      expect(screen.queryByRole("alertdialog")).toBeNull();
+      unmount();
+    }
+  });
+
+  it("blocks with a restarting message when down", () => {
+    render(<RestartOverlay store={storeAt("down")} />);
+    const dialog = screen.getByRole("alertdialog");
+    expect(dialog.textContent).toContain("Maestro is restarting");
+    expect(dialog.textContent).toContain("Waiting for the daemon to come back…");
+  });
+
+  it("counts attempts once more than one probe has failed", () => {
+    const store = storeAt("down");
+    store.setState({ attempts: 3 });
+    render(<RestartOverlay store={store} />);
+    expect(screen.getByRole("alertdialog").textContent).toContain("(3 attempts)");
+  });
+
+  it("says it is reloading when updating", () => {
+    render(<RestartOverlay store={storeAt("updating")} />);
+    const dialog = screen.getByRole("alertdialog");
+    expect(dialog.textContent).toContain("Maestro was updated");
+    expect(dialog.textContent).toContain("Reloading this page onto the new release…");
+  });
+
+  it("names the release from the store's build", () => {
+    const store = storeAt("down");
+    store.setState({ build: { commit: "f89be4a", commit_date: "2026-09-02T00:00:00Z", dirty: false } });
+    render(<RestartOverlay store={store} />);
+    expect(screen.getByRole("alertdialog").textContent).toContain("f89be4a · 2026-09-02");
+  });
+
+  it("clears the moment the store reports the link is back", () => {
+    const store = storeAt("down");
+    const { rerender } = render(<RestartOverlay store={store} />);
+    expect(screen.queryByRole("alertdialog")).not.toBeNull();
+    store.setState({ state: "online" });
+    rerender(<RestartOverlay store={store} />);
+    expect(screen.queryByRole("alertdialog")).toBeNull();
+  });
+
+  it("swallows keyboard shortcuts while blocking, but not once cleared", () => {
+    const store = storeAt("down");
+    const handler = vi.fn();
+    window.addEventListener("keydown", handler);
+    const { rerender } = render(<RestartOverlay store={store} />);
+
+    fireEvent.keyDown(window, { key: "n" });
+    expect(handler).not.toHaveBeenCalled();
+
+    store.setState({ state: "online" });
+    rerender(<RestartOverlay store={store} />);
+    fireEvent.keyDown(window, { key: "n" });
+    expect(handler).toHaveBeenCalledTimes(1);
+
+    window.removeEventListener("keydown", handler);
+  });
+});

web/src/components/daemon/RestartOverlay.tsx

@@ -0,0 +1,62 @@
+// React port of ../../restartOverlay.ts. Blocking overlay for a daemon that
+// went away — see docs/RESTART.md.
+//
+// Rendered while the link is `down` or `updating`, gone the moment it is
+// back. It owns no polling of its own: `DaemonLink` polls, this only renders
+// what the store reports.
+
+import { useEffect } from "react";
+import { useStore } from "zustand";
+
+import { formatRelease, type DaemonLinkStoreApi } from "./DaemonLink.js";
+
+export interface RestartOverlayProps {
+  /** The same store a sibling `DaemonLink` drives. */
+  store: DaemonLinkStoreApi;
+}
+
+/** The poll runs while this is up; saying how many tries have gone by is what
+ *  separates "still restarting" from "wedged" for someone staring at it. */
+function attemptText(attempts: number): string {
+  if (attempts <= 1) return "Waiting for the daemon to come back…";
+  return `Waiting for the daemon to come back… (${attempts} attempts)`;
+}
+
+function swallow(e: KeyboardEvent): void {
+  e.stopImmediatePropagation();
+}
+
+export function RestartOverlay({ store }: RestartOverlayProps) {
+  const state = useStore(store, (s) => s.state);
+  const build = useStore(store, (s) => s.build);
+  const attempts = useStore(store, (s) => s.attempts);
+
+  const blocking = state === "down" || state === "updating";
+
+  // The app's shortcuts live on `window`, so hiding the UI behind a backdrop
+  // does not stop them: `n` would still open a spawn form against a daemon
+  // that is not there. Swallow keys in the capture phase until the link is
+  // back, without `preventDefault` so the browser's own chords still work.
+  useEffect(() => {
+    if (!blocking) return;
+    window.addEventListener("keydown", swallow, true);
+    return () => window.removeEventListener("keydown", swallow, true);
+  }, [blocking]);
+
+  if (!blocking) return null;
+
+  const updating = state === "updating";
+
+  return (
+    <div className="modal-backdrop restart-backdrop">
+      <div className="modal restart-modal" role="alertdialog" aria-modal="true" aria-labelledby="restart-title">
+        <div className="restart-spinner" aria-hidden="true" />
+        <h2 id="restart-title">{updating ? "Maestro was updated" : "Maestro is restarting"}</h2>
+        <p className="restart-detail">
+          {updating ? "Reloading this page onto the new release…" : attemptText(attempts)}
+        </p>
+        <p className="restart-release">{formatRelease(build)}</p>
+      </div>
+    </div>
+  );
+}

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
-// (docs/PREVIEW.md) is issue #251; sidebar drag-to-dock and the mobile
+// React port of multiview.ts's tab + split tree (issue #249) plus its preview
+// 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/useHashRouter.test.tsx

@@ -0,0 +1,82 @@
+// @vitest-environment happy-dom
+import { act, cleanup, render, waitFor } from "@testing-library/react";
+import { afterEach, beforeEach, describe, expect, it } from "vitest";
+
+import { useHashRouter, type Route, type UseHashRouterResult } from "./useHashRouter.js";
+
+let controls: UseHashRouterResult;
+
+function Probe() {
+  controls = useHashRouter();
+  return <div>{controls.route.kind === "attach" ? `attach:${controls.route.sessionId}` : "catalog"}</div>;
+}
+
+beforeEach(() => {
+  location.hash = "";
+});
+
+afterEach(cleanup);
+
+describe("useHashRouter", () => {
+  it("starts on catalog for an empty hash", () => {
+    const view = render(<Probe />);
+    expect(view.container.textContent).toBe("catalog");
+  });
+
+  it("resolves #/attach/:id to the attach route", () => {
+    location.hash = "#/attach/s1";
+    const view = render(<Probe />);
+    expect(view.container.textContent).toBe("attach:s1");
+  });
+
+  it("decodes an encoded session id", () => {
+    location.hash = `#/attach/${encodeURIComponent("weird id/x")}`;
+    const view = render(<Probe />);
+    expect(view.container.textContent).toBe("attach:weird id/x");
+  });
+
+  it("falls back to catalog for an unknown route", () => {
+    location.hash = "#/nonsense";
+    const view = render(<Probe />);
+    expect(view.container.textContent).toBe("catalog");
+  });
+
+  it("follows a hashchange fired outside the hook", async () => {
+    const view = render(<Probe />);
+    // happy-dom dispatches `hashchange` on a real timer, like a browser — not
+    // synchronously with the assignment.
+    location.hash = "#/attach/s9";
+    await waitFor(() => expect(view.container.textContent).toBe("attach:s9"));
+  });
+
+  it("navigate updates the hash and the route", async () => {
+    const view = render(<Probe />);
+    act(() => controls.navigate({ kind: "attach", sessionId: "s2" } satisfies Route));
+    expect(location.hash).toBe("#/attach/s2");
+    await waitFor(() => expect(view.container.textContent).toBe("attach:s2"));
+  });
+
+  it("navigate back to catalog sets the root hash", async () => {
+    location.hash = "#/attach/s1";
+    const view = render(<Probe />);
+    act(() => controls.navigate({ kind: "catalog" }));
+    expect(location.hash).toBe("#/");
+    await waitFor(() => expect(view.container.textContent).toBe("catalog"));
+  });
+
+  it("navigating to the already-current route still updates state", () => {
+    location.hash = "#/attach/s1";
+    const view = render(<Probe />);
+    act(() => controls.navigate({ kind: "attach", sessionId: "s1" } satisfies Route));
+    expect(view.container.textContent).toBe("attach:s1");
+  });
+
+  it("stops listening after unmount", async () => {
+    const view = render(<Probe />);
+    view.unmount();
+    location.hash = "#/attach/s3";
+    // No assertion beyond "doesn't throw" — a leaked listener would only ever
+    // surface as noise in a later test's act() warnings.
+    await new Promise((r) => setTimeout(r, 0));
+  });
+});

web/src/hooks/useHashRouter.ts

@@ -0,0 +1,47 @@
+// React port of main.ts's `parseRoute`/`dispatch` pair (issue #240). Same two
+// routes, same hash shape (`#/`, `#/attach/:id`) — kept so a deep link or a
+// bookmark from the vanilla shell still resolves.
+
+import { useCallback, useEffect, useState } from "react";
+
+export type Route = { kind: "catalog" } | { kind: "attach"; sessionId: string };
+
+function parseHash(hash: string): Route {
+  const raw = hash.replace(/^#/, "");
+  if (raw === "" || raw === "/") return { kind: "catalog" };
+  const m = raw.match(/^\/attach\/([^/?#]+)$/);
+  if (m) return { kind: "attach", sessionId: decodeURIComponent(m[1]) };
+  return { kind: "catalog" };
+}
+
+function routeToHash(route: Route): string {
+  return route.kind === "catalog" ? "#/" : `#/attach/${encodeURIComponent(route.sessionId)}`;
+}
+
+export interface UseHashRouterResult {
+  route: Route;
+  navigate(route: Route): void;
+}
+
+export function useHashRouter(): UseHashRouterResult {
+  const [route, setRoute] = useState<Route>(() => parseHash(location.hash));
+
+  useEffect(() => {
+    const onHashChange = () => setRoute(parseHash(location.hash));
+    window.addEventListener("hashchange", onHashChange);
+    return () => window.removeEventListener("hashchange", onHashChange);
+  }, []);
+
+  const navigate = useCallback((next: Route) => {
+    const hash = routeToHash(next);
+    // Setting an unchanged hash never fires `hashchange`, so the state update
+    // that would otherwise ride on that event has to happen here instead.
+    if (location.hash === hash) {
+      setRoute(next);
+    } else {
+      location.hash = hash;
+    }
+  }, []);
+
+  return { route, navigate };
+}

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/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;
+      },
     };
   });
 }