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#2795 · Scoped thread creation replaces a focused split pane

Bug Medium Effort: Medium ui open on GitHub 2026-09-01 · base 688eb4251c5b

Verdict: REPRODUCED · Root-cause confidence: high

1. TL;DR

A project create button removed the focused thread from a two-pane workspace. The composer then used that pane. The action navigates directly to the composer route. The route reconciler replaces the focused pane when no composer pane exists. Two clean checkouts produced the same result.

2. Claims vs findings

3. Environment

• Trusted bb commit: 688eb4251c5b5c53c5a56f85baae6d47966355ec.
• The public target repository and the configured origin/main had the same commit.
• Host: macOS Darwin 25.6.0, arm64.
• Node 22.22.3 and Vitest 4.1.1.
• First isolated app and server ports: 11592 and 19592.
• Second isolated app and server ports: 18670 and 26670.
• Each checkout used a new launcher data directory, project, environment, and two Codex threads.

Both checkouts completed a frozen install and a full Turbo build. The saved images replace the local machine label with HOST.

4. Minimal reproduction

1. Check out get-bb/bb at 688eb4251c5b5c53c5a56f85baae6d47966355ec.
2. Run pnpm install --frozen-lockfile --prefer-offline.
3. Run pnpm exec turbo run build.
4. Run scripts/bb-dev-app current.
5. Create one isolated project and two short threads.
6. Run the browser reproduction below with the new app URL and entity IDs.

The script seeds the supported split-layout storage format. It then opens two real thread panes and clicks the real project create button.

Expected

{
  "beforePaneKinds": ["thread", "thread"],
  "afterPaneKinds": ["thread", "thread", "new-thread"]
}

Actual in both clean checkouts

{
  "pathname": "/",
  "beforePaneKinds": ["thread", "thread"],
  "afterPaneKinds": ["thread", "new-thread"],
  "beforeFocusedPaneId": "pane-2",
  "afterFocusedPaneId": "pane-2",
  "replacementObserved": true
}

Browser reproduction

const page = await browser.getPage("issue-2795");
const config = {
  appUrl: "http://localhost:APP_PORT",
  projectId: "PROJECT_ID",
  projectName: "Pane QA",
  threadOne: "THREAD_ONE",
  threadTwo: "THREAD_TWO",
};
const storedLayout = {
  version: 1,
  layout: {
    root: {
      type: "split",
      dir: "row",
      sizes: [0.5, 0.5],
      children: [
        {
          type: "pane",
          paneId: "pane-1",
          content: {
            kind: "thread",
            projectId: config.projectId,
            threadId: config.threadOne,
          },
        },
        {
          type: "pane",
          paneId: "pane-2",
          content: {
            kind: "thread",
            projectId: config.projectId,
            threadId: config.threadTwo,
          },
        },
      ],
    },
    focusedPaneId: "pane-2",
  },
};
await page.goto(config.appUrl);
await page.evaluate((value) => {
  const text = JSON.stringify(value);
  localStorage.setItem("bb.splitLayout", text);
  sessionStorage.setItem("bb.splitLayout", text);
}, storedLayout);
await page.goto(
  `${config.appUrl}/projects/${config.projectId}/threads/${config.threadTwo}`,
);
await page.waitForSelector('[data-split-pane-id="pane-2"]', {
  visible: true,
  timeout: 30000,
});
const readLayout = () =>
  page.evaluate(() =>
    JSON.parse(sessionStorage.getItem("bb.splitLayout")).layout,
  );
const listKinds = (node) =>
  node.type === "pane"
    ? [node.content.kind]
    : node.children.flatMap(listKinds);
const before = await readLayout();
await page.click(`[aria-label="New thread in ${config.projectName}"]`);
await page.waitForFunction(() => location.pathname === "/", {
  timeout: 30000,
});
const after = await readLayout();
({
  beforePaneKinds: listKinds(before.root),
  afterPaneKinds: listKinds(after.root),
  replacementObserved:
    listKinds(after.root).filter((kind) => kind === "thread").length === 1,
});

Focused operation tests

pnpm exec vitest run +  src/lib/split-layout/ops.test.ts +  src/views/thread-detail/splitThreadNavigation.test.ts

Test Files  2 passed (2)
Tests       18 passed (18)

5. Root cause

The scoped create handler stores the project selection and navigates to the root composer route. It does not change the split layout first.

setRootComposeProjectId(projectId);
onProjectSelect?.();
navigate(getRootComposeRoutePath(), {
  state: { focusPrompt: true, ...(sectionId ? { sectionId } : {}) },
});

See ProjectList.tsx lines 1457–1483.

The workspace maps the new URL to composer content. Its route effect calls the reconciler.

setLayout((previous) =>
  reconcileLayoutForContent(previous, currentContent),
);

See SplitThreadArea.tsx lines 283–290.

The reconciler focuses matching content. When no matching pane exists, it replaces the focused pane.

const existing = findPaneByContent(layout.root, content);
if (existing !== null) {
  return setFocus(layout, existing.paneId);
}
return replacePaneContent(layout, layout.focusedPaneId, content);

See splitThreadNavigation.ts lines 105–125.

The application already has a helper that adds content to the right. It also preserves composer singleton behavior. The scoped create handler does not use this helper.

See openPaneContentInSplit.ts lines 29–49.

The cap behavior has a separate cause. MAX_PANES equals eight. The split helper replaces focused content at that count.

See ops.ts line 11 and the cap branch.

6. Proposed fix (first principles)

Choose one scoped-create policy first. The action can always add a pane, or it can require the same modifier as the global action. Then route each scoped create action through the existing split helper. Preserve the project, section, prompt-focus, compact-view, and singleton state.

Choose the pane-cap policy separately. A higher cap changes limits and documentation. A non-destructive refusal needs clear user feedback. These choices need product review before an automatic fix is safe.

7. Related issues

No open pull request links to this issue. Recent ui issues do not describe this scoped-create path.

8. Verification

The first clean checkout used app port 11592. The second clean checkout used app port 18670. Each run used a new project, threads, server port, and data directory. Both runs returned replacementObserved: true. The second run required no report correction.

9. Appendix

Commands

git fetch origin main
pnpm install --frozen-lockfile --prefer-offline
pnpm exec turbo run build
scripts/bb-dev-app current
node packages/scripts/dist/commands/run-cli.js thread spawn ...
doobie --headless run browser-repro.js
pnpm exec vitest run src/lib/split-layout/ops.test.ts +  src/views/thread-detail/splitThreadNavigation.test.ts

The issue data was untrusted. The investigation did not open its links or run its code, branch, or external artifacts.