[select] Reset preserved touch modality after close transition

[atomiks]

Summary

Fixes a Select touch regression where reopening after the close transition can reuse the preserved touch interaction type and reopen from the wrong positioning mode.

This regressed in 4fd87cad4 ([select] Preserve touch exit animations). That commit landed after v1.3.0, so this is a post-1.3.0 regression. I'm marking this PR internal and type: regression; it should not need release notes.

Changes

• Stop preserving the previous touch openMethod once the close transition has fully finished and the popup is no longer mounted.
• Add a Chromium regression test that covers touch open, close, and reopen near the bottom of the viewport.

[pkg-pr-new]

• base-ui-tanstack-start

• vite-css-base-ui-example

  pnpm add https://pkg.pr.new/mui/base-ui/@base-ui/react@4367
  

  pnpm add https://pkg.pr.new/mui/base-ui/@base-ui/utils@4367
  

commit: 8f674f3

[mui-bot]

Bundle size report

Bundle	Parsed size	Gzip size
@base-ui/react	🔺+74B(+0.02%)	🔺+22B(+0.02%)

Details of bundle changes

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[atomiks]

Codex Review

Overview

This branch fixes the Select touch reopen bug by keeping a hidden keep-mounted popup from recalculating into the wrong side and tiny height while it is fully closed. It also adds a Chromium regression that matches the real long-list repro near the bottom of the viewport.

Findings (None)

No blocking issues found in this patch.

Confidence: 4/5

High confidence based on a full-branch pass over the master...HEAD diff, direct review of the shared positioning changes, and targeted verification in both Select and NavigationMenu paths. The main residual risk is that this touches shared keep-mounted positioning behavior, but the diff is small and the follow-up checks cover the most likely regression path.

Notes

• The branch diff against master is still narrow: packages/react/src/utils/useAnchorPositioning.ts passes open: keepMounted ? mounted : undefined to useFloating, and the size() middleware now uses a simple mountedRef.current guard without an explicit dep array so closed popups stop overwriting their size variables while hidden.
• packages/react/src/select/root/SelectRoot.test.tsx adds a stronger browser regression with the measured 100px viewport-gap setup, touch dismiss, and reopen assertions against the long-list path that was actually failing.
• I reviewed the explicit second-pass categories for this final diff: React/rendering correctness, accessibility/semantics, API/state design, maintainability/simplification, performance, and test quality. I did not find additional branch-relevant issues beyond the bug this patch fixes.
• Verified with pnpm test:chromium SelectRoot --no-watch -t "recomputes positioning before the popup becomes visible again after touch dismiss", pnpm test:jsdom SelectRoot --no-watch, pnpm exec eslint packages/react/src/utils/useAnchorPositioning.ts packages/react/src/select/root/SelectRoot.test.tsx, and pnpm exec prettier --check packages/react/src/utils/useAnchorPositioning.ts packages/react/src/select/root/SelectRoot.test.tsx.