Fix horizontal tab strip insertion positions

[austinywang]

Summary

• update bonsplit to use one pane-local drop destination across the horizontal tab strip
• compute insertion indices from live tab-frame midpoints so every between-tab slot is reachable
• preserve exact insertion behavior in translated split panes and for cross-pane moves
• keep tab sizing and overflow clipping unchanged

Upstream implementation: manaflow-ai/bonsplit#202

Regression coverage

The bonsplit PR preserves the required red/green history:

1. failing pointer-to-index regression tests
2. the AppKit drop destination, shared mutation path, and behavior tests

Follow-up review coverage rejects stale pasteboard-only transfers that cannot complete, validates model/visual frame order, and exercises translated AppKit hit testing.

Testing

• arch -arm64 "$(xcrun --find swift)" test in vendor/bonsplit
• 216 XCTest cases and 7 Swift Testing cases passed
• no new Swift warnings observed
• cloud visual proof was attempted twice, but neither pool assigned a runner within 15 minutes:
  • macOS 26: https://github.com/manaflow-ai/cmux-loader/actions/runs/31148006838
  • macOS 15: https://github.com/manaflow-ai/cmux-loader/actions/runs/31148877428

Review follow-up

• removed sorting from the drag-update hot path; insertion lookup is linear over already pane-ordered frames
• changed the resolver from static helper APIs to an instance collaborator
• require an actionable cross-pane fallback handler before advertising a pasteboard-only move

Issue: #9746

Closes #9746

[coderabbitai]

📝 Walkthrough

Walkthrough

The PR updates the vendor/bonsplit submodule reference from one commit to another. No exported or public declarations change.

Changes

Bonsplit vendor update

Layer / File(s)	Summary
Update submodule commit reference vendor/bonsplit	The submodule reference changes to commit 7d2aa113169d0183b00cae0ec4d97e3a1605b102.

Estimated code review effort: 1 (Trivial) | ~2 minutes

Suggested reviewers: lawrencecchen, azooz2003-bit

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Important

Pre-merge checks failed

Please resolve all errors before merging. Addressing warnings is optional.

❌ Failed checks (2 errors, 1 inconclusive)

Check name	Status	Explanation	Resolution
Cmux Algorithmic Complexity	❌ Error	New production code sorts all live tab frames in TabDropIndexResolver.swift:41 on every AppKit draggingUpdated call (TabBarDropDestinationNSView.swift:101-125); tabs are an unbounded array and no c...	Cache the pane's visually ordered frame midpoints when geometry changes, then use a linear lookup during draggingUpdated; otherwise add a measured, explicit tab-count bound and benchmark.
Cmux No Ambient Global State	❌ Error	New Sources/Bonsplit/Internal/Views/TabDropIndexResolver.swift:4 defines a type whose only behavior is static insertionIndex APIs at lines 10 and 23, creating a static-helper namespace.	Move insertion behavior to an instance-based, constructable TabDropIndexResolver injected into TabBarDropDestinationNSView, or use private/fileprivate file-scope pure helpers.
Cmux Swift Blocking Runtime	❓ Inconclusive	Placeholder while gathering evidence.	Continue repository inspection.

✅ Passed checks (22 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	The Bonsplit update addresses precise tab insertion, all tab-group indices, and consistent behavior across panes and splits while preserving out-of-scope behavior.
Out of Scope Changes check	✅ Passed	The changes are limited to the Bonsplit update for horizontal tab reordering and do not alter tab sizing or overflow clipping.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Cmux Swift Actor Isolation	✅ Passed	Bonsplit PR marks new AppKit/SwiftUI drop types and handler @MainActor; the pure resolver is unisolated, and no new Sendable reference or background UI access appears.
Cmux Browser Automation Off-Main	✅ Passed	The PR changes only the vendor/bonsplit gitlink; TerminalController.swift, the execution policy, and policy tests are unchanged, so no browser socket automation routing is introduced.
Cmux Expensive Synchronous Load	✅ Passed	The Bonsplit diff adds only @MainActor tab-drop UI/controller logic; its two JSONDecoder calls parse bounded TabTransferData from the drag pasteboard, with no agent-history files, scans, or disk lo...
Cmux Cache Substitution Correctness	✅ Passed	The diff only updates the Bonsplit submodule; its affected path uses live tab geometry for transient drop UI and in-memory reordering, not persistence, history, undo, or durable snapshots.
Cmux No Hacky Sleeps	✅ Passed	The PR changes only the vendor/bonsplit submodule pointer; no TypeScript, JavaScript, shell, or runtime-script diff introduces a sleep or fixed delay.
Cmux Swift Concurrency	✅ Passed	The diff changes only the vendor/bonsplit submodule pointer; no cmux-owned Swift files or app concurrency patterns are changed. Third-party code is outside this check.
Cmux Swift @Concurrent	✅ Passed	The Bonsplit update adds synchronous @MainActor drop views/handlers and removes synchronous main-queue bridging; it adds no async, nonisolated async, or @concurrent work.
Cmux Swift Package Boundaries	✅ Passed	The diff changes only the mode-160000 vendor/bonsplit gitlink; no app Swift or package files change. The boundary rule explicitly allows vendored code.
Cmux Swiftpm Lockfiles	✅ Passed	PR changes only the vendored vendor/bonsplit submodule pointer; no cmux SwiftPM or Xcode package files changed, and Bonsplit preserves its upstream Package.resolved ignore policy.
Cmux Swift Logging	✅ Passed	The Bonsplit patch adds no print, debugPrint, dump, NSLog, ad hoc logging, or Logger constants; it removes the prior DEBUG NSLog/dlog statements from TabBarView.
Cmux User-Facing Error Privacy	✅ Passed	The submodule diff adds tab-drop handling and tests only; review found no new user-facing errors, alerts, raw upstream messages, or sensitive diagnostics.
Cmux Full Internationalization	✅ Passed	The submodule diff changes tab-drop logic and tests only; it adds no production user-facing text or catalog/message files. Added literals are test names, fixtures, and diagnostics.
Cmux Swiftui State Layout	✅ Passed	The Bonsplit diff adds an NSViewRepresentable/AppKit drop bridge and reuses existing @Observable state; it adds no banned state, GeometryReader, lazy-row store reference, or render-time write.
Cmux Architecture Rethink	✅ Passed	The diff centralizes tab-strip drag handling in one AppKit destination and shared controller path; the pure midpoint resolver states its invariant, with no new sleeps, polling, locks, observers, or...
Cmux Swift Auxiliary Window Close Shortcuts	✅ Passed	The PR only updates the Bonsplit gitlink; the referenced diff changes tab-drop views, handlers, index logic, and tests. It adds no standalone cmux window or close-shortcut code.
Cmux Source Artifacts	✅ Passed	The only changed path is the existing vendor/bonsplit gitlink, updated for the stated product fix; no artifact, scratch directory, or generated file entered source control.
Cmux No Test Or Debug Seam In Production Source	✅ Passed	The Bonsplit production diff adds no DEBUG/test-only seams or test-named members; new helpers are called by production drop handling, and tests use @testable import.
Title check	✅ Passed	The title clearly and concisely describes the primary change to horizontal tab strip insertion positions.
Description check	✅ Passed	The description explains the change, motivation, regression coverage, testing results, and linked issue, but omits the template checklist and demo video section.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch issue-9746-horizontal-tab-insert

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[austinywang]

Review follow-up on the current bonsplit head 4da56ab:

• Algorithmic complexity: removed the per-draggingUpdated sort. The destination supplies frames in pane-model order, the resolver validates model/visual ordering in O(n), then performs a linear midpoint scan.
• Ambient global state: TabDropIndexResolver is now an instance collaborator owned by TabBarDropDestinationNSView; its insertion APIs are no longer static.
• Pasteboard fallback: a no-live-state transfer advertises .move only for a cross-pane source with an installed external handler.

The follow-up behavior tests cover unordered frame rejection, same-pane stale transfers, and cross-pane transfers without a handler. Local validation is 216 XCTest cases plus 7 Swift Testing cases, all passing with no warnings observed.