Restore the four-button pane tab bar in narrow panes

[austinywang]

The pane tab bar still differed from 0.64.25 after #17395 and #17396. Bonsplit 920b936 ("fix: collapse tab actions in narrow panes", Sep 30) collapsed the four action buttons into one … menu in any pane narrower than 520pt. That covers most split panes.

This PR bumps vendor/bonsplit from 64ac6d4 to 8bc81c8 (manaflow-ai/bonsplit#273), which removes that collapse:

• deletes isNarrowPane, splitActionButtonCount and collapsedSplitButtonMenu
• the lane always lays out every configured button again

The bump moves forward from the current pin and does not include bonsplit #272.

Audit against v0.64.25: splitButtonsOnHover is still false in cmux, and SplitActionButton.defaults is unchanged. After this PR, no other Bonsplit or cmux change since v0.64.25 alters which pane tab bar buttons are shown.

Verification: the Bonsplit package tests could not run locally because the cmux-lawrence-2 build-host alias is missing on this machine (REPAIR.md row 105). The Bonsplit and cmux CI on these PRs is the compile and test signal.

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Summary by cubic

Restores the four-button pane tab bar in narrow panes by bumping vendor/bonsplit to 8bc81c8 (manaflow-ai/bonsplit#273). The previous pin collapsed the pane tab bar's action buttons into a single ellipsis menu in any pane narrower than 520pt; the new pin always lays out every configured button again.

Verification:

• The bump moves forward from 64ac6d4 and does not include bonsplit Follow up PR 242: refresh browser under-page background on theme updates #272.
• splitButtonsOnHover remains false and SplitActionButton.defaults is unchanged, so no other change since v0.64.25 affects which pane tab bar buttons show.
• Bonsplit package tests could not run locally because the cmux-lawrence-2 build-host alias is missing; Bonsplit and cmux CI on the PRs provide the compile and test signal.

^{Written for commit 59c5780. Summary will update on new commits.}

Summary by CodeRabbit

• Chores
  • Updated a bundled third-party component. No user-visible changes are specified.

[github-actions]

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[austinywang]

Subagent correctness review: no correctness issues found. Source and history review only; nothing was built or run.

• Bonsplit 8bc81c8 leaves no references to the removed narrow-pane symbols in Sources or Tests, and none on the cmux side. splitButtonChrome and tabBarLayout are back to the v0.64.25 form, and no localized string is left orphaned.
• Between faf8418 (the v0.64.25 pin) and 8bc81c8, the only width-based change to which buttons show was 920b936/b281035, and this PR removes it. 98c35a8 (splitButtonsOnHover) has no effect because cmux sets that flag to false. The other commits change icon caching, accessibility labels, or tab-strip scrolling and hover, not which buttons appear.
• Tests: fullActionLaneWidth = 600 still renders the full lane. The two tests removed from TabBarLayoutFeedbackTests only covered the deleted helpers.
• cmux side: the Revert compact pane tab bar menus #17395 revert is clean and the default buttons are unchanged. One difference from v0.64.25 is by design: fix: hide browser affordances while the browser is disabled (#10866) #13023 hides the browser button when the browser is turned off, and hides flagged actions when their flags are off. All four buttons show with default settings.
• The submodule moves forward by exactly one commit (64ac6d4 → 8bc81c8) and does not include bonsplit#272.

Resolution: no changes needed.

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[github-actions]

Merge receipt for 59c578095a, merged 2026-10-05 15:29:35 UTC

• Not verified at merge: ci-status (not reported), macOS compile admission (in progress), guards (18) (in progress), swift-package-tests (in progress)
• Verified: backend migrations applied, CI fast guards, Fast static checks, GhosttyKit release check, plan, Web complexity, web-validation
• Skipped by policy: admission-placement, apply-production, apply-staging, browser, Claude wrapper regressions, Dogfood build #​${{ github.event.pull_request.number }}, full-suite-coverage, remote-daemon, suite-coverage, web, web-build, web-database-tests, and 1 more
• Full suite: runs on main after merge.

Labeled merged-unverified: if main breaks near this merge, look here first.