Fix overflowing tab selection scroll and chrome alignment

[austinywang]

Summary

• update bonsplit to use one MainActor AppKit geometry authority for tab frames, viewport scrolling, and selection chrome
• make the selected-tab visible rect exclude the trailing action lane, shifting right-edge tabs far enough left to expose their close buttons
• align the blue active indicator with the same right-edge fade as the selected tab content
• keep reveal intent pending until the selected item container reports its final live frame
• preserve programmatic reveal ownership through layout-driven clip-view bounds changes, while handing ownership back when live user scrolling begins
• remove competing SwiftUI proxy scrolling and queued clip-view corrections

Paired component PR

• Fix tab overflow selection geometry bonsplit#183

Regression coverage

• Bonsplit 033997e adds failing lifecycle tests for late scroll-view attachment and SwiftUI restoring a stale clip offset
• Bonsplit f647ae3 preserves pending reveal intent through attachment/layout and makes those lifecycle regressions pass

Verification

• arch -arm64 swift test in vendor/bonsplit: 197 tests passed
• tagged cmux Debug build succeeded and was launched with ./scripts/reload.sh --tag issue-8069-tab-overflow-scroll --launch
• python3 scripts/swift_file_length_budget.py was run and reports five pre-existing over-budget cmux files unchanged by this pointer-only parent diff

Localization audit

No user-facing strings were added or changed, so no English/Japanese catalog entries are required.

Addresses #8069

Summary by CodeRabbit

• Chores
  • Updated an integrated third-party component to a newer revision.
  • Maintenance update only, with no intentional end-user feature changes or behavior modifications.
  • No action is required from users.

[vercel]

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Project	Deployment	Actions	Updated (UTC)
cmux	Ready	Preview, Comment	Jul 14, 2026 7:41pm
cmux-staging	Building	Preview, Comment	Jul 14, 2026 7:41pm

[coderabbitai]

📝 Walkthrough

Walkthrough

The vendor/bonsplit git submodule pointer is updated from commit a0400b3 to f647ae3.

Changes

Bonsplit submodule update

Layer / File(s)	Summary
Advance Bonsplit revision vendor/bonsplit	Updates the submodule reference to commit f647ae39b34383808648b68dd43090143c6cd7cb.

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

Possibly related issues

• Overflowed horizontal tab bar scrolls to the wrong place; active-tab indicator misaligned/clipped #8069 — The new Bonsplit revision may include fixes for tab-bar scrolling and selection geometry.

Possibly related PRs

• manaflow-ai/cmux#4121 — Updates the same vendor/bonsplit submodule.
• manaflow-ai/cmux#4438 — Updates the same vendor/bonsplit submodule.
• manaflow-ai/cmux#6812 — Updates the same vendored Bonsplit dependency.

Suggested reviewers: lawrencecchen, azooz2003-bit

🚥 Pre-merge checks | ✅ 25

✅ Passed checks (25 passed)

Check name	Status	Explanation
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.
Cmux Swift Actor Isolation	✅ Passed	Changes are confined to a MainActor AppKit tab-geometry registry and tests; no new service protocols, Sendable refs, or background UI access were introduced.
Cmux Swift Blocking Runtime	✅ Passed	The vendored commit adds notification-driven scroll reconciliation; scans of changed files found no semaphores, waits, sleeps, main.sync, or locks, and only existing tests use RunLoop timing.
Cmux Browser Automation Off-Main	✅ Passed	Only vendor/bonsplit submodule pointer changed; no edits to the rule-scoped browser automation files or tests, so no off-main routing issue is introduced.
Cmux Expensive Synchronous Load	✅ Passed	Parent diff is only a vendor/bonsplit gitlink bump; no Swift source changed in the main repo, so this sync-load rule isn’t implicated.
Cmux Cache Substitution Correctness	✅ Passed	Change only adjusts transient tab scroll/geometry logic with live-scroll observers and freshness checks; no persistence/history/undo/snapshot cache substitution.
Cmux No Hacky Sleeps	✅ Passed	Only vendor/bonsplit submodule pointer changed; no TS/JS/shell/runtime-script sleeps or timing hacks were introduced. Swift changes are out of this check's scope.
Cmux Algorithmic Complexity	✅ Passed	f647 adds direct map lookups and one-pass observer iteration; no nested scans, repeated sorts/filters, or per-target rescans were introduced.
Cmux Swift Concurrency	✅ Passed	Diff only updates vendor/bonsplit submodule pointer; no cmux-owned Swift code or new concurrency patterns were introduced.
Cmux Swift @Concurrent	✅ Passed	No changed Swift concurrency annotations or actor-hopping issues; only UI-bound sync AppKit geometry updates and a main-queue test hop were added.
Cmux Swift File And Package Boundaries	✅ Passed	Only a vendored submodule pointer changed under vendor/bonsplit; the boundary rule explicitly allows vendored code and no cmux Swift source grew or mixed responsibilities.
Cmux Swiftpm Lockfiles	✅ Passed	Only vendor/bonsplit gitlink changed; it's a vendored third-party submodule, and no cmux-owned .gitignore, Package.swift, or cmux.xcodeproj package-reference changes occurred.
Cmux Swift Logging	✅ Passed	Parent diff only bumps vendor/bonsplit as a gitlink; no Swift source or logging statements were added or changed.
Cmux User-Facing Error Privacy	✅ Passed	The PR only updates the vendor/bonsplit submodule pointer; no user-facing error/alert/recovery text changed in the parent diff, and the vendored change is layout-focused.
Cmux Full Internationalization	✅ Passed	Only the vendor/bonsplit gitlink changed; no Swift text, catalogs, web messages, or Info.plist entries were modified.
Cmux Swiftui State Layout	✅ Passed	PASS: the changed bonsplit commits only add AppKit registry/bridge updates and tests; no new ObservableObject, @Published, GeometryReader, or render-time state writes appear.
Cmux Architecture Rethink	✅ Passed	Pointer-only bonsplit bump; the parent diff adds no Swift code and the described change is a local correctness fix with clear ownership/invariants, not a timing/split-lifecycle patch.
Cmux Swift Auxiliary Window Close Shortcuts	✅ Passed	Only the vendor/bonsplit gitlink changed; the outer diff adds no Swift NSWindow/WindowGroup code or cmux window-ID changes.
Cmux Source Artifacts	✅ Passed	Only vendor/bonsplit changed, and it is a deliberate submodule pointer update, not a local/generated artifact.
Cmux No Test Or Debug Seam In Production Source	✅ Passed	The PR only bumps vendor/bonsplit as a submodule gitlink; no production Swift Sources files or test/debug seams were added in this repo diff.
Cmux No Ambient Global State	✅ Passed	The diff only updates instance methods in TabBarItemGeometryRegistry.swift; no new free funcs, globals, static-only namespaces, or singletons were added.
Title check	✅ Passed	The title clearly matches the main change: fixing tab overflow scroll and chrome alignment.
Description check	✅ Passed	The description covers summary, verification, and regression coverage, with only minor template sections like demo video and checklist omitted.

✨ 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-8069-tab-overflow-scroll

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[greptile-apps]

Greptile Summary

This PR updates bonsplit’s overflowing tab selection behavior. The main changes are:

• Uses one MainActor AppKit authority for tab geometry and scrolling.
• Keeps selected tabs and close buttons clear of the trailing action lane.
• Aligns the active indicator with the tab’s right-edge fade.
• Waits for the selected item’s live frame before completing reveal.

Confidence Score: 5/5

This looks safe to merge.

• No blocking issues found in the updated code.

Important Files Changed

Filename	Overview
vendor/bonsplit	Updates the bonsplit submodule revision containing the tab geometry, overflow scrolling, reveal lifecycle, and selection chrome changes.

_{Reviews (3): Last reviewed commit: "Update bonsplit tab scroll lifecycle" | Re-trigger Greptile}