Defer browser view hierarchy mutations to prevent Auto Layout recursion

[lawrencecchen]

Summary

• macOS 26 Tahoe enforces stricter Auto Layout validation, crashing with EXC_BREAKPOINT when addSubview/removeFromSuperview is called during a layout pass
• Wraps attach/detach/dismantle view hierarchy mutations in scheduleDeferredHierarchyMutation (DispatchQueue.main.async) with generation guards to prevent stale mutations
• Adds regression test that rapidly switches between browser and terminal workspaces 120 times

Closes #653

Test plan

• Switch between browser-active and terminal workspaces rapidly on macOS 26
• Verify no crash (was EXC_BREAKPOINT/SIGTRAP previously)
• Verify browser panels still attach/detach correctly during normal tab switching
• Verify DevTools restoration still works after workspace switch

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Project	Deployment	Actions	Updated (UTC)
cmux	Building	Preview, Comment	Feb 28, 2026 1:30am

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📥 Commits

Reviewing files that changed from the base of the PR and between c5f7496 and 14ad5a1.

📒 Files selected for processing (3)

• Sources/Panels/BrowserPanelView.swift
• tests/test_browser_panel_stability.py
• tests_v2/test_browser_panel_stability.py

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch issue-653-autolayout-crash

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[greptile-apps]

Greptile Summary

This PR successfully addresses macOS 26 Tahoe's stricter Auto Layout validation by deferring view hierarchy mutations (addSubview/removeFromSuperview) using DispatchQueue.main.async. The implementation uses generation guards to prevent stale mutations and properly cancels pending operations during state transitions.

Key changes:

• Added scheduleDeferredHierarchyMutation helper that wraps hierarchy changes in async blocks with cancellation support
• Deferred all attach, detach, and dismantle operations that modify the view hierarchy
• Generation counter increments prevent stale deferred mutations from executing after coordinator state changes
• Comprehensive regression test validates the fix with 120 rapid workspace switches

Implementation quality:

• Thread safety: All mutations properly dispatched to main queue with DispatchQueue.main.async (follows project threading guidelines)
• Memory safety: Weak references to views prevent retain cycles, coordinator strongly captured to maintain generation guard integrity
• Cancellation: Deferred mutations cancelled appropriately when scheduling retries, entering portal mode, or dismantling
• Testing: Regression test thoroughly exercises the crash path with rapid workspace switching

Confidence Score: 4/5

• This PR is safe to merge with minimal risk after testing on macOS 26
• The implementation is solid with proper generation guards, thread safety, and weak references. The one minor suggestion is for consistency in the retry path. The fix directly addresses a critical crash and includes comprehensive regression testing.
• Verify Sources/Panels/BrowserPanelView.swift works correctly on macOS 26 Tahoe by running the regression test and manual workspace switching

Important Files Changed

Filename	Overview
Sources/Panels/BrowserPanelView.swift	Added deferred hierarchy mutation mechanism to prevent Auto Layout recursion crashes on macOS 26. Wraps view attach/detach operations in DispatchQueue.main.async with generation guards.
tests/test_browser_panel_stability.py	Added regression test that rapidly switches between browser-active and terminal workspaces 120 times to verify the Auto Layout fix works correctly.
tests_v2/test_browser_panel_stability.py	Duplicate of tests/test_browser_panel_stability.py with identical workspace switching regression test.

_{Last reviewed commit: 14ad5a1}

[greptile-apps]

_{3 files reviewed, 1 comment}

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[greptile-apps]

Additional Comments (1)

Sources/Panels/BrowserPanelView.swift
For consistency with immediate attach path (line 3561), consider wrapping in scheduleDeferredHierarchyMutation to ensure deferred execution even in retry path.

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