Forward tab drag threshold events

[austinywang]

Summary

• keep AppKit as the native tab-drag lifecycle owner
• forward the threshold-crossing mouse-drag event after starting the native session
• let SwiftUI cancel the pending tab tap/press before AppKit owns terminal mouse-up

Regression coverage

Commit 6c436f2 adds a focused test that fails against the previous behavior because the threshold event is swallowed. Commit c39140f forwards the event and makes the test pass.

Related to manaflow-ai/cmux#9521 and manaflow-ai/cmux#9787.

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Summary by cubic

Forward the threshold-crossing mouse-drag to SwiftUI when starting the native AppKit tab-drag session so SwiftUI cancels the pending tab tap/press. Prevents swallowed events and sticky drag latches; aligns with manaflow-ai/cmux#9521 and manaflow-ai/cmux#9787.

• Bug Fixes
  • Start AppKit tab-drag on threshold and forward that same event instead of consuming it, letting SwiftUI fail the press before mouse-up.
  • Added TabBarDragEventRoutingTests to cover the regression.

^{Written for commit c39140f. Summary will update on new commits.}

Summary by CodeRabbit

• Bug Fixes
  • Improved tab dragging so pending tab presses are correctly canceled when a drag begins.
  • Ensured the initial drag event is forwarded reliably, allowing native dragging to start smoothly.
• Tests
  • Added coverage for tab drag event routing and drag initiation.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro Plus

Run ID: ff5e5c49-aabc-4d96-a5e6-3cf748e61e4d

📥 Commits

Reviewing files that changed from the base of the PR and between cfe006e and c39140f.

📒 Files selected for processing (2)

• Sources/Bonsplit/Internal/Views/TabBarView.swift
• Tests/BonsplitTests/TabBarDragEventRoutingTests.swift

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📝 Walkthrough

Walkthrough

Tab drag handling now forwards the threshold-crossing drag event after native drag startup. The tab drag view and handler are internal. New AppKit tests verify event forwarding and drag initiation.

Changes

Tab drag event routing

Layer / File(s)	Summary
Forward threshold-crossing drag events Sources/Bonsplit/Internal/Views/TabBarView.swift	TabBarDragAndHoverView and handleTabDragEvent are internal. The handler returns the original drag event after invoking onBeginTabDrag.
Validate tab drag routing Tests/BonsplitTests/TabBarDragEventRoutingTests.swift	The test suite builds an AppKit tab-bar hierarchy, simulates mouse events, and verifies event forwarding and drag initiation for the registered tab.

Estimated code review effort: 2 (Simple) | ~10 minutes

Possibly related PRs

• manaflow-ai/bonsplit#202: Modifies related tab drag and drop-destination behavior in TabBarView.swift.
• manaflow-ai/bonsplit#205: Modifies tab-drag event handling and native AppKit drag lifecycle.

Suggested reviewers: lawrencecchen, azooz2003-bit

🚥 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: forwarding tab drag threshold events.
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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