Fix split divider drag tracking during multi-split resize

[austinywang]

Summary

• make divider hit testing use the same expanded effective rect as the split view
• treat divider boundary points as inside so drag tracking latches reliably
• write divider position updates synchronously to avoid stale ratio replay during fast drags

Verification

• consumed from cmux tagged Debug build during multi-split resize debugging

Notes

• no local tests run

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Summary by cubic

Fixes unreliable divider drag tracking during multi-split resizes. Expands the hit area and writes divider updates synchronously to prevent missed drags and snap-backs.

• Bug Fixes
  • Match the divider’s expanded hit rect with the split view and treat boundary points as inside to latch drags reliably.
  • Update divider position synchronously (no deferred Task) to avoid stale ratio replays during fast drags.

^{Written for commit 0e7b0b8. Summary will update on new commits.}

Summary by CodeRabbit

• Bug Fixes
  • Improved divider hit-detection accuracy for more precise boundary recognition during interactive resizing.
  • Enhanced divider position updates to provide more responsive behavior during drag operations.

[coderabbitai]

Caution

Review failed

The pull request is closed.

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: a9a92517-389f-4e44-bea1-29f1f86c5294

📥 Commits

Reviewing files that changed from the base of the PR and between aff9f70 and 0e7b0b8.

📒 Files selected for processing (1)

• Sources/Bonsplit/Internal/Views/SplitContainerView.swift

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📝 Walkthrough

Walkthrough

Modified hit-testing and state update logic in the split container divider handler. Added an inclusive boundary check helper function, adjusted divider hit rect inset from -4 to -5 pixels, and replaced asynchronous state updates with synchronous direct assignments in the drag delegate callback.

Changes

Cohort / File(s)	Summary
Divider Hit-Testing & State Management Sources/Bonsplit/Internal/Views/SplitContainerView.swift	Added dividerHitRectContains(_:rect:) helper for inclusive boundary checks using explicit min/max comparisons. Expanded divider hit rect inset to -5 and switched to custom hit-test function. Removed async Task wrapper around divider position persistence; state updates now execute synchronously in delegate callback. Adjusted DEBUG logging placement accordingly.

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~20 minutes

Poem

🐰✨ A rabbit hops with precision keen,
Where boundaries blur at the in-between.
No async dance, just sync and true,
Five pixels inset, the cleaner view.
Hit-testing refined, the divider now gleams! 🎉

✨ 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 fix-multi-split-divider-drag

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[cubic-dev-ai]

No issues found across 1 file

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[greptile-apps]

Greptile Summary

This PR fixes multi-split divider drag tracking with three coordinated changes: expanding the hit rect from -4 to -5 to match the existing effectiveRect/additionalEffectiveRect delegate expansion, replacing NSRect.contains (exclusive max edge) with a custom inclusive comparison dividerHitRectContains(_:rect:) to latch boundary-point drags, and removing a Task { @MainActor in } wrapper so divider position writes happen synchronously on the already-main-thread delegate callback — preventing stale ratio replays during fast drags.

Confidence Score: 5/5

Safe to merge — all three fixes are narrowly targeted and consistent with the surrounding delegate pattern.

No P0/P1 issues found. The -4→-5 alignment closes a hit-area mismatch between checkDividerDrag and the two AppKit delegate methods (effectiveRect/additionalEffectiveRect) that already used -5. The inclusive boundary check (>=/<=) directly fixes the known AppKit edge case where a point reported exactly on maxX/maxY would fail NSRect.contains. Removing the Task { @mainactor in } wrapper is safe because NSSplitViewDelegate callbacks always execute on the main thread; the isSyncingProgrammatically guard already prevents re-entrant feedback loops, and synchronous writes are consistent with the non-drag path that already calls syncPosition and onGeometryChange? synchronously.

No files require special attention.

Important Files Changed

Filename	Overview
Sources/Bonsplit/Internal/Views/SplitContainerView.swift	Three targeted fixes: hit rect aligned to -5 expansion, inclusive boundary hit testing, synchronous divider position writes

Sequence Diagram

sequenceDiagram
    participant User as User (Mouse)
    participant AppKit as NSSplitView (AppKit)
    participant Delegate as SplitCoordinator
    participant State as splitState

    User->>AppKit: leftMouseDown / leftMouseDragged
    AppKit->>Delegate: splitViewWillResizeSubviews
    Delegate->>Delegate: checkDividerDrag(event)<br/>hitRect = dividerRect.insetBy(-5,-5)<br/>dividerHitRectContains → inclusive boundary
    Delegate-->>Delegate: isDragging = true
    AppKit->>Delegate: splitViewDidResizeSubviews
    Delegate->>Delegate: wasDragging = isDragging && leftDown
    Delegate->>State: splitState.dividerPosition = normalizedPosition (synchronous, main thread)
    Delegate->>Delegate: onGeometryChange?(wasDragging)
    User->>AppKit: leftMouseUp
    AppKit->>Delegate: splitViewDidResizeSubviews
    Delegate->>Delegate: leftDown=false → wasDragging=false, isDragging=false
    Delegate->>Delegate: syncPosition(statePosition) — re-assert model

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