Fix tab drag never starting on second-click presses

[austinywang]

Problem

Regression from the native tab drag rewrite (6bee68f, after cmux 0.64.22): dragging a pane tab sporadically does nothing — no drag image, no session (manaflow-ai/cmux#10033).

Root cause: trackTabMouseDown only armed a drag when event.clickCount == 1. AppKit increments clickCount for any press that lands near the same point inside the double-click interval, so the extremely common flow click a tab to select it, then press again and drag it arrives with clickCount == 2 and the drag was silently never armed. Waiting ~500ms before dragging made it work again, which is why it felt sporadic and why clicking first "fixed" it. The 0.64.22 SwiftUI .onDrag path armed on any press.

Fix

Accept any clickCount when arming a pending tab drag (control-click still excluded for context menus). A stationary double-click is unaffected: the native session only begins after the pointer crosses the 4pt movement threshold.

Tests

Commit 1 adds secondClickOfDoubleClickStillArmsTabDrag (fails on main), commit 2 fixes it. Also carries the prior session's static-title-control coverage. Full suite: 22 tests green.

🤖 Generated with Claude Code

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Summary by cubic

Fixes a regression where dragging a tab after a quick second click wouldn’t start (fixes cmux #10033). Drags now arm on any click count and control hit-testing is tuned so labels don’t block drags while actionable controls keep ownership.

• Bug Fixes
  • Arm tab drags regardless of click count (control-click still excluded); stationary double-clicks remain unaffected by the movement threshold.
  • Recompute the dragging frame and image at threshold to reflect any UI changes before the drag starts.
  • Tighten native-control detection: static labels allow drags; editable/enabled controls keep presses. Begin callback now receives the threshold drag event; tests cover second-click drag, static label, and button ownership.

^{Written for commit 7679865. Summary will update on new commits.}

Summary by CodeRabbit

• Bug Fixes

  • Improved tab dragging for click-then-drag interactions, including double-click-initiated drags.
  • Preserved standard double-click behavior when the tab remains stationary.
  • Improved handling of buttons, text fields, and other interactive controls during tab dragging.
  • Refreshed tab visuals before starting a drag for more accurate drag previews.

• Tests

  • Added coverage for double-click dragging, static title controls, actionable buttons, and forwarded drag events.

[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: a91c708e-a771-4011-b9c8-1bc51c9c55ca

📥 Commits

Reviewing files that changed from the base of the PR and between 5b61b18 and 7679865.

📒 Files selected for processing (2)

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

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📝 Walkthrough

Walkthrough

Tab dragging now accepts double-click-counted presses, forwards the threshold-crossing event, refreshes the tab frame before drag creation, and filters native interactions by enabled, editable, or actionable controls. Tests cover event forwarding, click counts, static title controls, and buttons.

Changes

Tab drag routing

Layer / File(s)	Summary
Drag lifecycle and event forwarding Sources/Bonsplit/Internal/Views/TabBarView.swift, Tests/BonsplitTests/TabBarDragEventRoutingTests.swift	Drag tracking accepts non-control clicks with any click count. Pending state no longer stores the mouse-down event. Threshold crossing refreshes the tab frame and forwards the current event. Tests verify event forwarding and configurable click counts.
Native interaction filtering Sources/Bonsplit/Internal/Views/TabBarView.swift, Tests/BonsplitTests/TabBarDragEventRoutingTests.swift	Native interaction detection handles enabled buttons, editable text inputs, and enabled controls with targets and actions. Tests cover static title controls and actionable buttons.

Estimated code review effort: 3 (Moderate) | ~20 minutes

Possibly related PRs

• manaflow-ai/bonsplit#205: Introduced the native tab-drag initiation and event-routing logic refined here.
• manaflow-ai/bonsplit#207: Added related threshold-event routing that this change extends.

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 summarizes the main fix: tab dragging now starts when the second click is used to initiate a drag.
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-10033-second-click-drag

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