Make tab drag lifecycle cleanup synchronous

[austinywang]

Follow-up for manaflow-ai/cmux#9521.

• invoke drag lifecycle cleanup synchronously from AppKit main-thread callbacks
• make monitor registration cleanup survive controller teardown via isolated deinit
• assert app-resign teardown completes before notification posting returns

Validation: arch -arm64 swift test (217 XCTest + 7 Swift Testing passed).

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Summary by cubic

Make tab drag cleanup synchronous to prevent sticky drag latches and stale session callbacks. Event monitors and app-resign handling now end the drag immediately on main-thread callbacks and during controller teardown, addressing issue 9521.

• Bug Fixes
  • End the active drag generation synchronously from AppKit main-thread monitors (ESC, mouseUp, app resign) via MainActor.assumeIsolated (no deferred Task).
  • Make teardown synchronous during stop() and deinit: store monitor tokens as nonisolated(unsafe) and remove them via a static helper, avoiding an isolated-deinit runtime dependency.
  • Update test to assert immediate cleanup on app resign (no Task.yield).

^{Written for commit 6a9b467. Summary will update on new commits.}

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Review details

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro Plus

Run ID: 3918d3e9-183e-423a-be27-b8be3a555b5b

📥 Commits

Reviewing files that changed from the base of the PR and between aa16fb9 and 6a9b467.

📒 Files selected for processing (1)

• Sources/Bonsplit/Internal/Controllers/TabDragLifecycleMonitor.swift

📝 Walkthrough

Walkthrough

The tab drag lifecycle monitor centralizes AppKit observer cleanup, performs synchronous main-actor end handling, and removes deferred cleanup tasks. The app-resign test now verifies drag-state cleanup synchronously.

Changes

Tab drag lifecycle

Layer / File(s)	Summary
Observer registration ownership Sources/Bonsplit/Internal/Controllers/TabDragLifecycleMonitor.swift	The monitor stores AppKit observers in Registrations and removes them through removeAll().
Synchronous drag-end handling Sources/Bonsplit/Internal/Controllers/TabDragLifecycleMonitor.swift, Tests/BonsplitTests/BonsplitTests.swift	Event callbacks use MainActor.assumeIsolated. requestEnd() retains generation and stopped-state guards. Deinitialization and stop() remove registrations. The app-resign test checks cleanup without yielding.

Estimated code review effort: 3 (Moderate) | ~20 minutes

Possibly related PRs

• manaflow-ai/bonsplit#202: Related tab-drag lifecycle and cleanup behavior.

Suggested reviewers: lawrencecchen

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the main change: making tab drag lifecycle cleanup synchronous.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch issue-9521-sticky-drag-latches

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