[tabs][toolbar] Reduce bundle size

[atomiks]

Removes behavior-neutral dead weight from the Tabs and Toolbar entry points. The bundle bot is authoritative for the current size impact.

Runtime-only enum values are inlined and pinned by enum-sync tests, redundant context members are removed, duplicated lookup and direction logic is consolidated, and the composite substrate's IndexGuessBehavior representation is made tree-shakeable. Tabs, Toolbar, Toggle Group, and composite regressions cover the affected behavior.

[pkg-pr-new]

• base-ui-tanstack-start

• vite-css-base-ui-example

  pnpm add https://pkg.pr.new/mui/base-ui/@base-ui/react@5221
  

  pnpm add https://pkg.pr.new/mui/base-ui/@base-ui/utils@5221
  

commit: f260347

[code-infra-dashboard]

Bundle size

Bundle	Parsed size	Gzip size
@base-ui/react	▼-1.37KB(-0.30%)	▼-323B(-0.22%)

Details of bundle changes

Performance

Total duration: 1,198.31 ms -165.33 ms^(-12.1%) | Renders: 78 ⁽⁺⁰⁾ | Paint: 1,870.81 ms -240.25 ms^(-11.4%)

Test	Duration	Renders
Popover mount (300 instances)	58.67 ms ▼-21.16 ms(-26.5%)	1 (+0)
Checkbox mount (500 instances)	57.99 ms ▼-15.36 ms(-20.9%)	1 (+0)
Tooltip mount (300 contained roots)	45.91 ms ▼-13.13 ms(-22.2%)	1 (+0)

11 tests within noise — details

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Check out the code infra dashboard for more information about this PR.

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[Copilot]

Pull request overview

This PR reduces bundle size in the Tabs and Toolbar entrypoints by removing behavior-neutral indirections (enum/IIFE references, redundant context members) and by consolidating repeated lookup/direction logic, while keeping runtime behavior pinned by existing and new tests.

Changes:

• Toolbar: simplify separator orientation derivation, remove unused setItemMap from ToolbarRootContext, and rely on downstream defaults for optional props.
• Tabs: inline data attribute / CSS variable names (with a new test to keep them synced with docs enums), simplify tab activation/highlight wiring, and deduplicate tab element lookup + activation-direction computation.
• Composite internals: make IndexGuessBehavior tree-shakeable and trim small dead/default code paths.

Reviewed changes

Copilot reviewed 22 out of 22 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
packages/react/src/toolbar/separator/ToolbarSeparator.tsx	Simplifies derived separator orientation without enum indirection.
packages/react/src/toolbar/root/ToolbarRootContext.ts	Removes unused context member to trim toolbar entrypoint weight.
packages/react/src/toolbar/root/ToolbarRoot.tsx	Drops redundant context wiring and relies on downstream defaults for optional props.
packages/react/src/toolbar/link/ToolbarLink.test.tsx	Updates test context shape after context member removal.
packages/react/src/toolbar/input/ToolbarInput.tsx	Makes group context optional and deduplicates disabled-event prevention handlers.
packages/react/src/toolbar/input/ToolbarInput.test.tsx	Updates test context shape after context member removal.
packages/react/src/toolbar/group/ToolbarGroupContext.ts	Makes group context access always-optional to enable tree-shaking and simpler call sites.
packages/react/src/toolbar/group/ToolbarGroup.test.tsx	Updates test context shape after context member removal.
packages/react/src/toolbar/button/ToolbarButton.tsx	Makes group context optional and relies on useButton defaults for nativeButton.
packages/react/src/toolbar/button/ToolbarButton.test.tsx	Updates test context shape after context member removal.
packages/react/src/toggle-group/ToggleGroup.tsx	Makes toolbar group context optional to avoid unnecessary runtime branching.
packages/react/src/tabs/tab/TabsTab.tsx	Removes redundant list-context members, uses composite root highlight state, and calls onValueChange directly with guarded activation.
packages/react/src/tabs/root/TabsRootContext.ts	Changes panel registration to return an unsubscribe cleanup function.
packages/react/src/tabs/root/TabsRoot.tsx	Extracts shared tab lookup helper and consolidates activation-direction logic.
packages/react/src/tabs/root/stateAttributesMapping.ts	Inlines the emitted data attribute name for better tree-shaking.
packages/react/src/tabs/panel/TabsPanel.tsx	Simplifies panel list item registration and uses cleanup-returning panel registration.
packages/react/src/tabs/list/TabsListContext.ts	Removes dead/redundant context members to reduce tabs entrypoint size.
packages/react/src/tabs/list/TabsList.tsx	Stops re-exporting activation/highlight plumbing through list context; keeps it inside composite root usage.
packages/react/src/tabs/indicator/TabsIndicator.tsx	Inlines CSS variable names and slightly simplifies scale checks used for measurement.
packages/react/src/tabs/enumSync.test.tsx	Adds tests to keep docs enums and runtime inlined literals in sync without shipping production bytes.
packages/react/src/internals/composite/root/useCompositeRoot.ts	Removes small redundant defaults/returns and simplifies modifier-key iteration.
packages/react/src/internals/composite/list/useCompositeListItem.ts	Replaces enum with tree-shakeable const object + type alias for IndexGuessBehavior.

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