fix(server): a timed-out browser drag no longer exits the server

[ScottN-PV]

Problem

drag starts dragTo, then awaits the target's position and a cursor move before it awaits the drag. A drag that times out in that gap rejects with no handler attached, and the server's only unhandledRejection listener exits the process. That matches #17322: the server restarted and startup recovery cancelled the running threads.

Change

drag now handles the drag's outcome in the turn the drag starts, finishes the cursor move as before, then rethrows the drag's error. The server keeps running and the action fails with the drag's error. Ordering is unchanged: a failed drag still settles inside its action, and a cursor-move failure still takes precedence.

Scope and approval

Closes #17322. The triage comment confirms the unobserved rejection on main and suggests catching the drag promise at once and rethrowing after the cursor update, which is this change.

Verification

• New test in ServerBrowserPage.test.ts: a page whose drag fails at once while the target's position is still pending. No unhandledRejection fires, drag does not settle until the target's position arrives, and it then rejects with the drag's error. The test fails with main's ServerBrowserPage.ts.
• In apps/server: vp test run src/preview passes, 10 files, 127 tests. Server typecheck, vp lint, and vp fmt --check on the two changed files pass.
• Not reproduced against a live server.

Model: Claude Opus 5.5, Claude Fable 5.1 (review), GPT-6-Astra (review). Harness: Claude Code, Codex.

[macroscopeapp]

Approvability

Verdict: Approved at b3d6d43

Macroscope's review found this PR approvable — This is a focused bug fix that prevents a timed-out browser drag from causing an unhandled rejection and terminating the server, while preserving normal drag behavior and operation-level error reporting. A targeted regression test covers the failure timing, and no product defaults or static-analysis suppressions are changed.

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[coderabbitai]

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ℹ️ Recent review info

⚙️ Run configuration

• Configuration used: Path: .coderabbit.config.ts
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• Run ID: 219489d2-f38f-4d2a-9a2a-e7ce3820b4fa

📥 Commits

Reviewing files that changed from the base of the PR and between 5b78b34 and b3d6d43.

📒 Files selected for processing (2)

• apps/server/src/preview/ServerBrowserPage.test.ts
• apps/server/src/preview/ServerBrowserPage.ts

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📝 Walkthrough

Walkthrough

The drag operation now observes a rejected source.dragTo(target) promise while moving the pointer. It reports the captured error afterward. A regression test covers a timeout while the target position is pending.

Changes

Browser drag timeout handling

Layer / File(s)	Summary
Capture and report drag failures apps/server/src/preview/ServerBrowserPage.ts, apps/server/src/preview/ServerBrowserPage.test.ts	The drag operation records a rejection before moving the pointer and rethrows it afterward. The test checks that the operation waits for the target position and does not produce an unhandled rejection.

Priority: ➖ Normal

Estimated code review effort: 2 (Simple) | ~10 minutes

Change: Bug fix · Severity of issue fixed: Medium

Suggested reviewers: juliusmarminge

Merge Risk: ⚪ Minimal · up to b3d6d

The drag timeout is intended to return an error without exiting the server, and the reported targeted checks pass. No actionable merge-blocking risk is identified.

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check		The title clearly and concisely describes the primary fix: preventing a timed-out browser drag from terminating the server.
Description check		The description includes the required Problem, Change, Scope and approval, and Verification sections. It explains the failure mode, links the issue and approval comment, describes the focused test cov…
Linked Issues check		For directly linked issue #17322, drag attaches a rejection handler to source.dragTo(target, { timeout }) in the same turn that it starts the drag. It then completes target.boundingBox and the f…
Out of Scope Changes check		The reviewed changes are limited to ServerBrowserPage.ts and its focused drag regression test. The implementation and test directly address #17322. No unrelated production behavior or unrelated test…

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