fix(sidebar): order panels without reading a single tab title

[STRML]

Status: updated onto main (78f4c82504) as merge commit 25a2e642a5. Still a draft: it calls bonsplitController.tabIds(inPane:), which lands in manaflow-ai/bonsplit#233 and is not in the pinned submodule yet. Nothing else in the diff needs that PR.

Reviewer's guide

• Read Sources/Workspace.swift (sidebarOrderedPanelIds) and the new PaneSpatialOrder enum; the tree method delegates to it, so there is one implementation.
• Tests: Packages/macOS/CmuxPanes/Tests/CmuxPanesTests/SpatialOrderTests.swift.
• Skip the pbxproj line.

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Draft: blocked on manaflow-ai/bonsplit#233. This calls bonsplitController.tabIds(inPane:), which does not exist in the bonsplit commit currently pinned here. It will not compile until that PR merges and the submodule pointer moves. Opening it now so the cmux-side change is reviewable alongside it. The pointer bump is deliberately not in this diff.

Summary

sidebarOrderedPanelIds() reads every tab's title to produce a list of ids.

It asks bonsplit for tabs(inPane:) and builds a treeSnapshot(). Both read titles: Tab.init(from:) copies all fourteen TabItem properties, and the snapshot puts a title in every ExternalTab. Sidebar views call this from inside their bodies, so an animating tab title invalidates them for an ordering that never looks at a title.

allPaneIds already walks the tree first-then-second, exactly as ExternalTreeNode.orderedPaneIds does, so the snapshot was not buying the pane order either.

Change

The call site asks for tabIds(inPane:) and never mentions a tab. The ordering moves out of the ExternalTreeNode extension into PaneSpatialOrder, which takes pane ids directly; the tree method delegates to it, so there is still one implementation and one set of tests.

Measurement

From a 20.7s sample of a running cmux, main thread:

Frame	Time
sidebarOrderedPanelIds path	142.4 ms
VerticalTabsSidebar body	1058.6 ms (21.4%)

After, in a 16013-sample run with tab titles updating, sidebarOrderedPanelIds costs 8 samples (0.05%) and treeSnapshot does not appear on the main thread at all.

Two caveats on those numbers, since they are not a clean A/B. The before and after traces are from different sessions under different load, so treat the 142.4 ms against 8 samples as a direction rather than a ratio. The VerticalTabsSidebar figure is the whole body, which this change only partly accounts for; the rest is #5832.

Related

• perf: add tabIds(inPane:) so callers stop building tabs to read ids bonsplit#233 adds the method this depends on.
• fix(portal): repaint the divider overlay only when it could have changed #12009, fix(portal): ask bonsplit which pane owns a tab instead of scanning #10856, fix: keep spinner titles animating without waking the app on every frame #10351, fix(focus-history): resolve each history entry once per menu snapshot #10349, fix: bound the shared closed-item history at 100 records #10353 are the other idle-work fixes from the same profiles.
• Sidebar: VerticalTabsSidebar.body rebuilds ~5 O(N) dictionaries on every workspace state change (130+ workspaces, continuous agent-driven updates) #5832 covers the remaining VerticalTabsSidebar body cost.

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https://claude.ai/code/session_01Bj791kgph6Xk9CJf9c41Db

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[STRML]

Rebased onto main (78f4c82), no conflicts. Still a draft on purpose: it calls tabIds(inPane:), which lands in manaflow-ai/bonsplit#233 and is not in the pinned submodule yet, so it cannot compile until that merges and the pointer moves. Everything else in the diff is reviewable now; the description opens with a reviewer's guide. @lawrencecchen if bonsplit#233 looks good, this is the cmux-side half.

[STRML]

Superseded by #13931 (ac0ceae), which landed this exact mechanism on main 2026-09-24: SpatialPanelOrder in Packages/macOS/CmuxPanes/Sources/CmuxPanes/Geometry/SpatialPanelOrder.swift orders from bonsplitController.allPaneIds with no tree snapshot, and sidebarOrderedPanelIds() delegates to it. Upstream's paneTabs[paneId] ?? [] loop already exhibits this PR's one behavior #13931 had no test for (a pane id missing from paneTabs is skipped, not fatal), so the only thing left here is a duplicate.\n\nThanks to maintainers for merging the same fix; nothing left to carry.