Phase 1: tabbed workspaces

Each tab is an independent tile tree; PTYs in non-active tabs keep
running (render-all-panes with visibility:hidden on inactive layers
so xterm.js's fit() still sees valid dimensions and the existing
per-pane resize dedupe absorbs no-op SIGWINCHes).

workspace.json shape goes from a bare TreeNode to
`{ version: 2, workspaces: [{ id, name, tree }] }` with a legacy v1
auto-wrap migration (the old single tree becomes one tab named
"Default").

App.tsx wraps the old single-tree state in workspace-aware state
but keeps `tree` / `setTree` / `activeLeafId` / `setActiveLeafId` as
identity-stable derived wrappers (reading currentWorkspaceId from a
ref), so the bulk of App.tsx stays unchanged.

XtermPane's initial term.focus() now checks `visibility !== "hidden"`
on the container so a pane mounting inside a hidden tab on app boot
doesn't yank focus away from the active tab. The focus poller is
scoped to the active workspace layer for the same reason.

Shortcuts: Ctrl+T new tab, Ctrl+Shift+T close current (window.confirm
when there are live panes), Ctrl+PageDown/PageUp navigate, Ctrl+1..9
switch to tab N. README + help overlay auto-generated from
shortcuts.ts.

79/79 vitest pass (7 new envelope-migration cases). tsc -b clean.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/lib/layout/tree.test.ts

@@ -21,6 +21,10 @@ import {
   MAX_FONT_SIZE,
   serialize,
   deserialize,
+  serializeWorkspaces,
+  deserializeWorkspaces,
+  singletonEnvelope,
+  WORKSPACES_VERSION,
   presetSingle,
   presetTwoColumns,
   presetThreeColumns,
@@ -665,3 +669,82 @@ describe("serialize / deserialize", () => {
     expect(back.sshHostId).toBe("h-1");
   });
 });
+
+describe("workspaces envelope", () => {
+  it("roundtrips a multi-workspace envelope", () => {
+    const env = {
+      version: WORKSPACES_VERSION,
+      workspaces: [
+        { id: "w1", name: "alpha", tree: newLeaf({ distro: "Ubuntu" }) },
+        {
+          id: "w2",
+          name: "beta",
+          tree: newSplit("h", newLeaf({ label: "left" }), newLeaf()),
+        },
+      ],
+    };
+    const back = deserializeWorkspaces(serializeWorkspaces(env));
+    expect(back).toEqual(env);
+  });
+
+  it("returns null on invalid JSON", () => {
+    expect(deserializeWorkspaces("not json")).toBeNull();
+  });
+
+  it("returns null when version is wrong or workspaces is missing", () => {
+    expect(deserializeWorkspaces('{"version": 99, "workspaces": []}')).toBeNull();
+    expect(deserializeWorkspaces('{"version": 2}')).toBeNull();
+  });
+
+  it("returns null when an envelope has zero valid workspaces", () => {
+    expect(
+      deserializeWorkspaces('{"version": 2, "workspaces": [{"id": 1}]}'),
+    ).toBeNull();
+  });
+
+  it("migrates a legacy v1 bare-tree JSON into a single 'Default' workspace", () => {
+    const legacy = JSON.stringify({
+      kind: "split",
+      id: "s1",
+      orientation: "h",
+      ratio: 0.5,
+      a: { kind: "leaf", id: "a", distro: "Ubuntu" },
+      b: { kind: "leaf", id: "b", distro: "PowerShell" },
+    });
+    const env = deserializeWorkspaces(legacy);
+    expect(env).not.toBeNull();
+    expect(env!.version).toBe(WORKSPACES_VERSION);
+    expect(env!.workspaces.length).toBe(1);
+    expect(env!.workspaces[0].name).toBe("Default");
+    // Per-leaf legacy migration also applied — PowerShell sentinel mapped.
+    const tree = env!.workspaces[0].tree as SplitNode;
+    expect((tree.a as LeafNode).shellKind).toBe("wsl");
+    expect((tree.b as LeafNode).shellKind).toBe("powershell");
+    expect((tree.b as LeafNode).distro).toBeUndefined();
+  });
+
+  it("singletonEnvelope wraps a tree with a fresh workspace id", () => {
+    const t = newLeaf({ label: "only" });
+    const env = singletonEnvelope(t, "Main");
+    expect(env.workspaces.length).toBe(1);
+    expect(env.workspaces[0].name).toBe("Main");
+    expect(env.workspaces[0].tree).toBe(t);
+    expect(typeof env.workspaces[0].id).toBe("string");
+    expect(env.workspaces[0].id).not.toBe(t.id);
+  });
+
+  it("skips malformed workspaces but keeps the valid ones", () => {
+    const env = {
+      version: WORKSPACES_VERSION,
+      workspaces: [
+        { id: "ok", name: "alpha", tree: { kind: "leaf", id: "L" } },
+        { id: 42, name: "bad-id", tree: { kind: "leaf", id: "L2" } },
+        { id: "no-tree", name: "still-bad" },
+      ],
+    };
+    const back = deserializeWorkspaces(JSON.stringify(env));
+    expect(back).not.toBeNull();
+    expect(back!.workspaces.length).toBe(1);
+    expect(back!.workspaces[0].id).toBe("ok");
+  });
+});

src/lib/layout/tree.ts

@@ -703,6 +703,92 @@ export function deserialize(json: string): TreeNode | null {
   }
 }
 
+// ---- workspaces envelope ---------------------------------------------------
+
+/** One named tab in the tab strip. Each workspace owns its own tile tree;
+ *  leaf NodeIds remain globally unique across workspaces so the app-level
+ *  paneIdByLeaf map continues to work without partitioning. */
+export interface Workspace {
+  id: NodeId;
+  name: string;
+  tree: TreeNode;
+}
+
+/** Top-level persistence shape. `version` bumps when the envelope schema
+ *  changes; the v1 shape was a bare TreeNode at the JSON root, migrated
+ *  automatically by {@link deserializeWorkspaces}. */
+export interface WorkspacesEnvelope {
+  version: 2;
+  workspaces: Workspace[];
+}
+
+export const WORKSPACES_VERSION = 2 as const;
+
+/** Construct an envelope wrapping a single workspace with the given tree.
+ *  Used for first-launch and as the destination of the v1→v2 migration. */
+export function singletonEnvelope(tree: TreeNode, name = "Default"): WorkspacesEnvelope {
+  return {
+    version: WORKSPACES_VERSION,
+    workspaces: [{ id: newId(), name, tree }],
+  };
+}
+
+export function serializeWorkspaces(env: WorkspacesEnvelope): string {
+  return JSON.stringify(env);
+}
+
+/** Parse a persisted workspaces envelope. Accepts:
+ *  - Current shape: `{ version: 2, workspaces: [{ id, name, tree }] }`
+ *  - Legacy v1 shape: a bare {@link TreeNode} — wrapped as one workspace
+ *    named "Default" with a fresh id.
+ *  Per-leaf legacy migrations ({@link migrateLegacyLeaves}) still apply to
+ *  each workspace's tree. Returns null when the JSON is unrecognisable. */
+export function deserializeWorkspaces(json: string): WorkspacesEnvelope | null {
+  let parsed: unknown;
+  try {
+    parsed = JSON.parse(json);
+  } catch {
+    return null;
+  }
+
+  // v1: bare TreeNode at the root
+  if (isTreeNode(parsed)) {
+    return singletonEnvelope(migrateLegacyLeaves(parsed));
+  }
+
+  // v2: envelope
+  if (
+    typeof parsed === "object" &&
+    parsed !== null &&
+    (parsed as { version?: unknown }).version === WORKSPACES_VERSION &&
+    Array.isArray((parsed as { workspaces?: unknown }).workspaces)
+  ) {
+    const raw = (parsed as { workspaces: unknown[] }).workspaces;
+    const workspaces: Workspace[] = [];
+    for (const w of raw) {
+      if (
+        typeof w !== "object" ||
+        w === null ||
+        typeof (w as { id?: unknown }).id !== "string" ||
+        typeof (w as { name?: unknown }).name !== "string" ||
+        !isTreeNode((w as { tree?: unknown }).tree)
+      ) {
+        continue;
+      }
+      const tw = w as { id: string; name: string; tree: TreeNode };
+      workspaces.push({
+        id: tw.id,
+        name: tw.name,
+        tree: migrateLegacyLeaves(tw.tree),
+      });
+    }
+    if (workspaces.length === 0) return null;
+    return { version: WORKSPACES_VERSION, workspaces };
+  }
+
+  return null;
+}
+
 /** Sentinel used in pre-shellKind workspaces to mark PowerShell panes. */
 const LEGACY_POWERSHELL_DISTRO = "PowerShell";