fix(desktop): Branch button silently does nothing inside a branched chat tile

[alelpoan]

What does this PR do?

Fixes a dead-button UX bug: inside a session opened as a tile (i.e. a chat that was itself created via Branch in new chat), the Branch button on assistant replies is fully visible and looks clickable — but clicking it does absolutely nothing, with no error, no toast, no feedback of any kind.

Nested branching from a tile isn't supported by design (session-tile.tsx intentionally wires onBranchInNewChat to a no-op), but the button itself rendered unconditionally regardless of whether a real handler existed, so the "unsupported here" state was invisible to the user. This is the same class of problem as an earlier fixed bug where a dead Refresh button gave no feedback either.

Related Issue

Fixes #

Type of Change

• 🐛 Bug fix (non-breaking change that fixes an issue)
• ✨ New feature (non-breaking change that adds functionality)
• 🔒 Security fix
• 📝 Documentation update
• ✅ Tests (adding or improving test coverage)
• ♻️ Refactor (no behavior change)
• 🎯 New skill (bundled or hub)

Changes Made

• apps/desktop/src/components/assistant-ui/thread/assistant-message.tsx: the Branch button in AssistantActionBar now only renders when onBranchInNewChat is actually provided ({onBranchInNewChat && (...)}), matching the existing pattern already used for onDismissError and onRestoreToMessage — both of which are conditionally rendered for the same reason (per the comment in latest-actions.ts: presence, not just callability, is what these children gate on).
• apps/desktop/src/app/chat/session-tile.tsx: no longer passes a no-op onBranchInNewChat={() => undefined} for tiled/branched chats; the prop is simply omitted, so the button now doesn't render there at all instead of rendering dead.
• apps/desktop/src/app/chat/index.tsx: onBranchInNewChat on ChatViewProps is now optional (?), since omitting it is now a legitimate, intentional state rather than something every caller had to fake with a no-op.
• apps/desktop/src/app/contrib/latest-actions.ts: the latestChatActions passthrough wrapper now uses the existing latestOptional helper for onBranchInNewChat instead of an unconditional call, consistent with how onDismissError/onRestoreToMessage are already wrapped.
• apps/desktop/src/components/assistant-ui/thread/assistant-message.test.tsx (new): covers both states — button visible when a handler is passed, button absent when it isn't.

How to Test

1. Open a normal chat, send a message, hover an assistant reply — the Branch icon is visible and clickable, as before (no regression).
2. Use Branch in new chat to open a branched tab/tile.
3. Hover an assistant reply inside that branched tile — before this fix, the Branch icon was visible and clicking it did nothing; after the fix, the icon isn't shown at all for messages in a tile, so there's nothing misleading to click.
4. Run the new test file directly: npx vitest run src/components/assistant-ui/thread/assistant-message.test.tsx

Checklist

Code

• I've read the Contributing Guide
• My commit messages follow Conventional Commits (fix(scope):, feat(scope):, etc.)
• I searched for existing PRs to make sure this isn't a duplicate
• My PR contains only changes related to this fix/feature (no unrelated commits)
• I've run pytest tests/ -q and all tests pass
• I've added tests for my changes (required for bug fixes, strongly encouraged for features)
• I've tested on my platform: Windows 11

Documentation & Housekeeping

• I've updated relevant documentation (README, docs/, docstrings) — or N/A
• I've updated cli-config.yaml.example if I added/changed config keys — or N/A
• I've updated CONTRIBUTING.md or AGENTS.md if I changed architecture or workflows — or N/A
• I've considered cross-platform impact (Windows, macOS) per the compatibility guide — or N/A
• I've updated tool descriptions/schemas if I changed tool behavior — or N/A

Screenshots / Logs

[austinpickett]

Review — approved

Verified the fix end to end. It's a clean, minimal fix that extends the codebase's existing conventions rather than inventing anything.

What it does right

• AssistantActionBar now gates the Branch button on {onBranchInNewChat && (...)}, matching the exact pattern already used for onDismissError (assistant-message.tsx:114). Presence-gating is the established contract for these action-bar children.
• latest-actions.ts switches onBranchInNewChat to the existing latestOptional() helper, consistent with every other optional handler (onDismissError, onRestoreToMessage, onTranscribeAudio, ...).
• session-tile.tsx drops the () => undefined no-op instead of faking a handler, and ChatViewProps.onBranchInNewChat becomes optional to make omission a legitimate state. The chain is internally consistent.
• Tooltip copy.branchNewChat resolves to 'Branch in new chat', matching the test's getByRole name.

Verification performed

• mergeable: MERGEABLE / CLEAN — no conflicts.
• The 4 changed source files typecheck with zero errors.
• New test passes (2/2). Confirmed non-vacuous: reverting the conditional render makes the "button hidden when no handler" case fail as expected, then passes again once restored.

No blocking issues. LGTM.

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Reviewed by Hermes Agent