Fix re-entrant exclusive-access crash in drag handle hit test

[lawrencecchen]

Summary

Fixes #761.

SIGABRT crash caused by re-entrant calls to windowDragHandleShouldCaptureHit during the sibling hit-test walk. When sibling.hitTest() triggers a SwiftUI layout pass, AppKit calls back into the drag handle's hit resolution while the outer invocation is still walking siblings. This violates Swift's exclusive-access rules on SwiftUI view state.

The fix adds a module-level re-entrancy guard (_windowDragHandleIsResolvingSiblingHits) that bails out on nested calls to the sibling walk. Main-thread only, no lock needed.

Crash stack: DraggableView.hitTest -> windowDragHandleShouldCaptureHit -> sibling.hitTest -> SwiftUI body eval -> hitTest (re-entry) -> exclusive-access violation -> SIGABRT.

Test plan

• Added testDragHandleSiblingHitTestReentrancyDoesNotCrash regression test that simulates the re-entrant call path
• Existing drag handle tests continue to pass (same 4 pre-existing failures on main)
• Reproduced on macOS Sequoia 15.1.1 VM, deployed fix, verified crash no longer occurs

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Project	Deployment	Actions	Updated (UTC)
cmux	Building	Preview, Comment	Mar 3, 2026 3:01am

[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 b3f6f8c and 568da85.

📒 Files selected for processing (2)

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

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📝 Walkthrough

Walkthrough

A re-entrancy guard mechanism is added to WindowDragHandleView to prevent SwiftUI from re-entering the sibling-hit testing path, with an accompanying test that verifies the guard prevents crashes during re-entrant scenarios.

Changes

Cohort / File(s)	Summary
Re-entrancy Guard Sources/WindowDragHandleView.swift	Introduces private boolean flag _windowDragHandleIsResolvingSiblingHits to block re-entrant hit-test calls. Guard is set before sibling-walk begins and reset via defer. Early return with DEBUG log if re-entrancy detected.
Re-entrancy Test cmuxTests/CmuxWebViewKeyEquivalentTests.swift	Adds test helper class ReentrantSiblingView and test testDragHandleSiblingHitTestReentrancyDoesNotCrash to verify re-entrancy guard prevents crashes and yields false result during re-entrant hit-test attempts.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~8 minutes

Possibly related PRs

• Fix drag-handle crash on launch from stale foreign-window events #620: Modifies hit-testing logic in WindowDragHandleView.swift around windowDragHandleShouldCaptureHit and hit-test path.
• Fix exclusive-access crash in WindowDragHandleView hitTest #736: Addresses re-entrant SwiftUI access crashes in the same hit-testing code path with an alternative approach.

Poem

🐰 A guard stands tall at the gate so wise,
Preventing re-entry before SwiftUI cries,
With defer in hand, it resets with grace,
No crashes shall happen in this sacred space! ✨

🚥 Pre-merge checks | ✅ 2 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 25.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (2 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The pull request title 'Fix re-entrant exclusive-access crash in drag handle hit test' clearly and accurately describes the main change: fixing a re-entrancy crash in the drag handle hit-testing logic.

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✨ Finishing Touches

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🧪 Generate unit tests (beta)

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

---

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[cubic-dev-ai]

No issues found across 2 files

[greptile-apps]

Greptile Summary

Fixed a SIGABRT crash on macOS Sequoia caused by re-entrant calls to windowDragHandleShouldCaptureHit during sibling hit-test walks. When sibling.hitTest() triggers SwiftUI layout, AppKit can call back into the drag handle's hit resolution before the outer invocation completes, violating Swift's exclusive-access rules.

• Added _windowDragHandleIsResolvingSiblingHits guard flag set before the sibling walk
• Re-entrant calls now bail out early (return false) instead of crashing
• Used defer to ensure flag is always reset, even on early returns
• Added ReentrantSiblingView test helper and regression test validating the fix

Confidence Score: 5/5

• This PR is safe to merge with minimal risk
• Focused fix for a specific crash scenario with clear root cause analysis, proper guard implementation using defer pattern, and comprehensive regression test coverage that validates both outer and re-entrant call behavior
• No files require special attention

Important Files Changed

Filename	Overview
Sources/WindowDragHandleView.swift	Added module-level re-entrancy guard to prevent SIGABRT crash when sibling hit-test triggers SwiftUI layout
cmuxTests/CmuxWebViewKeyEquivalentTests.swift	Added regression test simulating re-entrant call path that previously caused crash

Sequence Diagram

sequenceDiagram
    participant User
    participant DraggableView
    participant Guard as Re-entrancy Guard
    participant Function as windowDragHandleShouldCaptureHit
    participant Sibling as ReentrantSiblingView
    
    User->>DraggableView: Mouse down event
    DraggableView->>Function: hitTest(point)
    Function->>Guard: Check _windowDragHandleIsResolvingSiblingHits
    Guard-->>Function: false (not set)
    Function->>Guard: Set flag = true
    
    Note over Function: Start sibling walk
    Function->>Sibling: hitTest(pointInSibling)
    
    Note over Sibling: Triggers SwiftUI layout
    Sibling->>Function: Re-entrant call (same drag handle)
    Function->>Guard: Check _windowDragHandleIsResolvingSiblingHits
    Guard-->>Function: true (already set!)
    Function-->>Sibling: Return false (bail out, no crash)
    
    Sibling-->>Function: Return nil
    Note over Function: Continue sibling walk
    Function->>Guard: defer resets flag = false
    Function-->>DraggableView: Return true (capture hit)

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_{Last reviewed commit: 568da85}