[Janpot]

Composite items rendered through useCompositeItem (Tabs, Toolbar, Radio Group, toggle groups, Navigation Menu, and the menubar's Menu triggers) learned their index only after mount, so every item rendered twice. They now guess it from render order with useCompositeListItem's existing guess option, as Menu and Combobox items already do; a wrong guess, for grouped or out-of-order items, is corrected in the commit before paint.

The smoke comment below doesn't list it: no difference is confidently past its 10% band. Locally against master, the smoke cases of the components it touches (30 rounds, 99% intervals):

	renders per mount	render	paint
RadioGroup	2 → 1	−15.4% to −9.9%	−9.6% to −2.1%
ToggleGroup	2 → 1	−24.0% to −13.2%	no change detected
Toolbar	2 → 2	−17.3% to −8.0% (nested update −69.1% to −50.3%, mount +0.0% to +9.0%)	no change detected
Tabs, Menubar, NavigationMenu	unchanged	no change detected	no change detected

Split out of #5974.

[pkg-pr-new]

• base-ui-tanstack-start

• vite-css-base-ui-example

  pnpm add https://pkg.pr.new/mui/base-ui/@base-ui/react@6094
  

  pnpm add https://pkg.pr.new/mui/base-ui/@base-ui/utils@6094
  

commit: 241dbba

[code-infra-dashboard]

Bundle size

Bundle	Parsed size	Gzip size
@base-ui/react	🔺+14B(0.00%)	🔺+7B(0.00%)

Details of bundle changes

Performance

No regressions in 36 benchmarks · ran 3m 58s · details

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