[Bug]: Creating a thread from a pull request does not link the pull request to the thread#15721

Triage

Thanks for tracing all three checkout paths, @gertzgal. I didn't find another issue covering this.

What I found

I checked main at 4ee6bfd. Checkout prepares the branch and points a thread at it, but it never writes a thread.pull-request.link.

• GitManager.preparePullRequestThread resolves and returns the pull request. It only uses threadId for the setup script and never dispatches a link. Creating a pull request from a thread does link it, via linkCreatedPullRequest, because that thread already exists.
• Every checkout caller keeps only branch and worktreePath and drops the resolved pull request. That includes PullRequestThreadDialog with handlePreparedPullRequestThread, PullRequestDetailPanel, and usePullRequestHandoffs. Several of these pass the id of a draft. Drafts are client state, and thread.pull-request.link loads an existing server thread, so linking at prepare time would have nothing to attach to. PersistedDraftThreadState has no pull request field.
• Once the draft becomes a thread, ThreadPullRequestService can store a discovered branchPullRequest, and badges fall back to it (useLinkedThreadPullRequest). The linked list, the pull-requests panel, snapshot sync, stack siblings, and watch all read links, though, so they stay empty until someone links manually.
• Ask and Explain don't check out and don't link either. Mobile has no checkout-from-pull-request path.

Likely fix area

• One option is to store the resolved pull request on the draft that receives the checkout. Then dispatch thread.pull-request.link when that draft becomes a server thread. All three web callers already have result.pullRequest.
• Today's link sources are manual, created, agent, stack, and stack-dismissed. A checkout link might want its own source.

A maintainer will decide on the fix direction.

Reproduced in the web dev app on b5a226e49c, using isolated state and a GitHub-backed project. The chat checkout dialog in Local mode and the PR detail panel checkout in a separate worktree both completed. After materializing each draft as a thread, the persisted thread had its PR branch and branchPullRequest reference, but pullRequests remained empty. The visible branch badge therefore does not establish an explicit link. macOS Electron has not been checked.

Fix submitted in #23458. The web draft retains the checkout PR and links each successfully started thread. Focused tests, web typecheck and lint pass. Runtime records cover the original failure and a persisted link after reload; matched browser recordings and Electron verification remain incomplete after environment interruptions.