[tabs] Fix suspended panel activation

[atomiks]

Fixes #4880

Opening a tab whose panel suspends no longer makes Tabs treat the temporarily empty tab registry as real tab removal.

Root cause

Suspense can clean up tab registration effects while keeping the previous tab nodes connected, so the automatic missing-tab fallback reset the value before the suspended panel could resume. The connectivity check has to run from the root effect after the DOM settles; during a real unmount, unregister can run before React detaches the DOM node.

Changes

• Track a previously registered tab element and only commit the missing-tab fallback when that element is disconnected.
• Add a React 19 Suspense regression test for opening a suspending panel with the boundary outside Tabs.Root.

[pkg-pr-new]

• base-ui-tanstack-start

• vite-css-base-ui-example

  pnpm add https://pkg.pr.new/mui/base-ui/@base-ui/utils@4903
  

  pnpm add https://pkg.pr.new/mui/base-ui/@base-ui/react@4903
  

commit: 165a731

[code-infra-dashboard]

Bundle size

Bundle	Parsed size	Gzip size
@base-ui/react	🔺+67B(+0.01%)	🔺+35B(+0.02%)

Details of bundle changes

Performance

Total duration: 1,258.66 ms +48.31 ms^(+4.0%) | Renders: 50 ⁽⁺⁰⁾ | Paint: 1,920.31 ms +93.43 ms^(+5.1%)

Test	Duration	Renders
Checkbox mount (500 instances)	123.43 ms 🔺+22.78 ms(+22.6%)	1 (+0)

11 tests within noise — details

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