fix(branch-selector): self-heal a stale cached default branch

Four reviewers (qa-swarm's paul + xp, plus the org Codex and veria-ai bots)
independently flagged that a stale cached default branch never self-corrects:
the picker auto-selects the cached "trunk", and the existing effect's
`!selectedBranch` guard then blocks re-selection once the live default arrives,
so a task could start on a branch that is no longer the default.

Track the branch we auto-selected in a ref and re-select when the default
changes out from under our own auto-pick — while never clobbering a branch the
user chose deliberately. Adds tests for both the self-heal and the
user-choice-preserved paths, and updates the TaskInput comment to match.

Generated-By: PostHog Code
Task-Id: 562fb90f-d823-425d-b53c-5f482b55a8b2

Author: pauldambra

packages/ui/src/features/git-interaction/components/BranchSelector.test.tsx

@@ -124,6 +124,84 @@ describe("BranchSelector cloud mode", () => {
     expect(screen.queryByRole("option", { name: "main" })).toBeNull();
   });
 
+  it("re-selects the default when a stale cached default is replaced by the live one", () => {
+    const onBranchSelect = vi.fn();
+    const { rerender } = renderInTheme(
+      <BranchSelector
+        repoPath="owner/repo"
+        currentBranch={null}
+        defaultBranch="master"
+        workspaceMode="cloud"
+        cloudBranches={[]}
+        cloudBranchesLoading={true}
+        cloudSearchQuery=""
+        selectedBranch={null}
+        onBranchSelect={onBranchSelect}
+        onCloudSearchChange={vi.fn()}
+      />,
+    );
+
+    // Auto-selected the (stale) cached default.
+    expect(onBranchSelect).toHaveBeenLastCalledWith("master");
+
+    // Parent commits that selection; then the live default arrives differing.
+    rerender(
+      <Theme>
+        <BranchSelector
+          repoPath="owner/repo"
+          currentBranch={null}
+          defaultBranch="main"
+          workspaceMode="cloud"
+          cloudBranches={["main"]}
+          cloudBranchesLoading={false}
+          cloudSearchQuery=""
+          selectedBranch="master"
+          onBranchSelect={onBranchSelect}
+          onCloudSearchChange={vi.fn()}
+        />
+      </Theme>,
+    );
+
+    expect(onBranchSelect).toHaveBeenLastCalledWith("main");
+  });
+
+  it("does not override a branch the user picked when the default later changes", () => {
+    const onBranchSelect = vi.fn();
+    const { rerender } = renderInTheme(
+      <BranchSelector
+        repoPath="owner/repo"
+        currentBranch={null}
+        defaultBranch="master"
+        workspaceMode="cloud"
+        cloudBranches={["master", "feature-x"]}
+        cloudBranchesLoading={false}
+        cloudSearchQuery=""
+        selectedBranch="feature-x"
+        onBranchSelect={onBranchSelect}
+        onCloudSearchChange={vi.fn()}
+      />,
+    );
+
+    rerender(
+      <Theme>
+        <BranchSelector
+          repoPath="owner/repo"
+          currentBranch={null}
+          defaultBranch="main"
+          workspaceMode="cloud"
+          cloudBranches={["main", "feature-x"]}
+          cloudBranchesLoading={false}
+          cloudSearchQuery=""
+          selectedBranch="feature-x"
+          onBranchSelect={onBranchSelect}
+          onCloudSearchChange={vi.fn()}
+        />
+      </Theme>,
+    );
+
+    expect(onBranchSelect).not.toHaveBeenCalled();
+  });
+
   it("surfaces the 'Use input as branch name' action when the typed value is new", async () => {
     const user = userEvent.setup();
     renderInTheme(

packages/ui/src/features/git-interaction/components/BranchSelector.tsx

@@ -144,8 +144,19 @@ export function BranchSelector({
   const isSelectionOnly = workspaceMode === "worktree" || isCloudMode;
   const displayedBranch = isSelectionOnly ? selectedBranch : currentBranch;
 
+  // The branch we auto-selected, so we can tell our own pick apart from one the
+  // user made. Lets us correct a stale default (e.g. a cached "trunk" that the
+  // live list later contradicts) without ever clobbering a deliberate choice.
+  const autoSelectedBranchRef = useRef<string | null>(null);
   useEffect(() => {
-    if (isSelectionOnly && defaultBranch && !selectedBranch && onBranchSelect) {
+    if (!isSelectionOnly || !defaultBranch || !onBranchSelect) return;
+    // Adopt the default when nothing is selected yet, or when the default has
+    // changed out from under a value we ourselves auto-selected — but leave a
+    // user's own selection alone.
+    const selectionIsOurs =
+      !selectedBranch || selectedBranch === autoSelectedBranchRef.current;
+    if (selectionIsOurs && selectedBranch !== defaultBranch) {
+      autoSelectedBranchRef.current = defaultBranch;
       onBranchSelect(defaultBranch);
     }
   }, [isSelectionOnly, defaultBranch, selectedBranch, onBranchSelect]);

packages/ui/src/features/task-detail/components/TaskInput.tsx

@@ -435,10 +435,10 @@ export function TaskInput({
   const liveCloudDefaultBranch = cloudBranchData?.defaultBranch ?? null;
   // Serve the persisted default branch until the live list resolves, so the
   // majority "start on trunk" case pre-selects trunk with zero wait on a cold
-  // start. The cached value is best-effort: `cloudDefaultBranch` switches to the
-  // live value the moment it arrives, but the picker's auto-select only fires
-  // while nothing is selected — so a default branch renamed since it was cached
-  // (rare) would leave the seeded name selected until the user picks another.
+  // start. The cached value is best-effort: if it's stale (a default branch
+  // renamed since it was cached), `cloudDefaultBranch` switches to the live
+  // value on arrival and BranchSelector re-selects it — as long as the user
+  // hasn't picked a branch of their own in the meantime.
   const cloudDefaultBranch =
     liveCloudDefaultBranch ??
     (selectedCloudRepository