Fix drag-handle crash on launch from stale foreign-window events

[lawrencecchen]

Summary

• Add window-identity check to windowDragHandleShouldCaptureHit so stale leftMouseDown events from other apps (Finder, Dock) during launch don't trigger the SwiftUI hierarchy walk while initial layout is mutating (exclusive-access violation)
• Add NSLock to WindowDragHandleBreadcrumbLimiter for thread safety
• Add regression test for nil/foreign/matching eventWindow scenarios
• Fix shouldApplyImmediateHostedStateUpdate test call sites (removed stale hostWindowAttached: param)

Closes #490

Test plan

• Launch app with mouse hovering over window area (was crash scenario)
• Launch from Dock, Spotlight, terminal
• Verify drag-handle still works for normal clicks after launch
• Run unit tests: xcodebuild test -only-testing:cmuxTests

Summary by CodeRabbit

• Bug Fixes

  • Improved thread-safety and memory management in window drag operations to enhance stability.
  • Enhanced hit-testing accuracy for drag capture across multiple window scenarios.

• Tests

  • Updated tests to validate window-aware drag behavior and reentrancy handling.

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Project	Deployment	Actions	Updated (UTC)
cmux	Building	Preview, Comment	Feb 27, 2026 7:20am

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

Configuration used: defaults

Review profile: CHILL

Plan: Pro

📥 Commits

Reviewing files that changed from the base of the PR and between e20d692 and 03fb930.

📒 Files selected for processing (2)

• Sources/WindowDragHandleView.swift
• cmuxTests/CmuxWebViewKeyEquivalentTests.swift

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📝 Walkthrough

Walkthrough

The changes add thread safety to drag-handle event processing via NSLock and refactor hit-test logic to be window-aware. A method is renamed to windowDragHandleShouldResolveActiveHitCapture with new parameters for event and drag-handle windows, and call-sites are updated accordingly. Cache pruning is extended, and tests are updated to exercise the new window-scoped behavior.

Changes

Cohort / File(s)	Summary
Thread safety and hit-test refactoring Sources/WindowDragHandleView.swift	Added NSLock for thread-safe access to lastEmissionByKey dictionary; extended pruning logic for stale entries when map exceeds 128 items. Renamed windowDragHandleShouldDeferHitCapture(for:) to windowDragHandleShouldResolveActiveHitCapture(for:eventWindow:dragHandleWindow:) with new window-identity and event-type guards. Updated all call-sites to pass eventWindow and dragHandleWindow parameters; adjusted internal references to use dragHandleWindow for consistency in suppression and depth checks.
Test signature alignment cmuxTests/CmuxWebViewKeyEquivalentTests.swift	Updated test calls to pass eventWindow: NSWindow? parameter to windowDragHandleShouldCaptureHit. Adjusted assertions to verify window-aware reentrant hit resolution and nested window scenarios. Removed hostWindowAttached: Bool parameter expectations from shouldApplyImmediateHostedStateUpdate calls.

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~20 minutes

Poem

🐰 A rabbit hops through locks so tight,
Where windows dance and events ignite,
No more crashes in the race—
Thread-safe guards now hold their place! 🔒✨

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 6.25% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 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 fix: preventing drag-handle crashes caused by stale foreign-window events, which aligns with the primary objective from issue #490.
Linked Issues check	✅ Passed	All code objectives from issue #490 are addressed: window-identity check in windowDragHandleShouldCaptureHit [#490], NSLock for thread safety in breadcrumb limiter [#490], and regression tests for nil/foreign/matching eventWindow scenarios [#490].
Out of Scope Changes check	✅ Passed	All changes directly support the linked issue objectives: hit-test refactoring for window identity checks, thread-safe breadcrumb limiting with NSLock, stale entry pruning, test updates including hostWindowAttached parameter removal, and new eventWindow parameter added consistently across call sites.

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✨ Finishing Touches

• 📝 Generate docstrings (stacked PR)
• 📝 Generate docstrings (commit on current branch)

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch issue-490-drag-handle-crash

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[greptile-apps]

Greptile Summary

Fixes crash during app launch when mouse hovers over window area. The core issue was stale leftMouseDown events from external apps (Finder, Dock) triggering SwiftUI hierarchy traversal while initial layout was mutating, causing exclusive-access violations.

Key changes:

• Added window-identity check in windowDragHandleShouldResolveActiveHitCapture to filter out foreign-window events before hierarchy walk
• Added NSLock to WindowDragHandleBreadcrumbLimiter.shouldEmit for thread-safe dictionary access
• Added comprehensive regression test covering nil/foreign/matching event window scenarios
• Cleaned up stale hostWindowAttached: parameter from test call sites

The fix is narrowly scoped and preserves existing behavior for legitimate same-window events while safely rejecting stale cross-app events during launch.

Confidence Score: 5/5

• Safe to merge - well-tested crash fix with clear scope
• The changes are defensive (preventing crashes), well-tested with regression coverage, and use proper synchronization primitives. The window-identity check is a sound approach to filter stale events without affecting normal operation.
• No files require special attention

Important Files Changed

Filename	Overview
Sources/WindowDragHandleView.swift	Added window-identity check to prevent stale foreign-window events from triggering SwiftUI hierarchy walk during launch, plus NSLock for thread-safe breadcrumb throttling
cmuxTests/CmuxWebViewKeyEquivalentTests.swift	Added regression test for foreign-window event handling and fixed stale test call sites by removing obsolete hostWindowAttached parameter

_{Last reviewed commit: 03fb930}