[menu] Fix race conditions

[atomiks]

Found a couple of these working on #3783

1. Sometimes the highlight would remain stuck on an item despite the pointer leaving. onFocus gets called after onPointerLeave is called with null incorrectly.
2. Sometimes a submenu would incorrectly close on mouseleave despite being hovered over already (where it should remain open).

Both are hard to reproduce; more frequent with CPU slowdown

[pkg-pr-new]

• vite-css-base-ui-example

  pnpm add https://pkg.pr.new/mui/base-ui/@base-ui/react@3821
  

  pnpm add https://pkg.pr.new/mui/base-ui/@base-ui/utils@3821
  

commit: 909603c

[mui-bot]

Bundle size report

Bundle	Parsed size	Gzip size
@base-ui/react	▼-43B(-0.01%)	▼-6B(0.00%)

Details of bundle changes

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[greptile-apps]

Greptile Summary

Fixed two race conditions in the menu component's hover and focus handling:

• useListNavigation: Added synchronous focus clearing in onPointerLeave to prevent highlight from getting stuck on items when the pointer leaves. Previously, onFocus could fire after onPointerLeave had already cleared the index, incorrectly re-highlighting the item.

• MenuPositioner: Removed the logic that re-enabled hover on parent menus when child submenus closed. This logic was causing submenus to incorrectly close on mouseleave despite being hovered over, because the hoverEnabled state could toggle while hovering submenu triggers, temporarily unregistering event listeners during the cleanup/re-execution phase.

• MenuSubmenuTrigger: Changed arrow function to method shorthand syntax (no functional change)

These race conditions were difficult to reproduce under normal conditions but became more obvious with CPU slowdown or when dealing with many submenu triggers.

Confidence Score: 5/5

• This PR is safe to merge with minimal risk
• The changes are surgical fixes targeting specific race conditions without introducing new functionality. The added enqueueFocus(null, { sync: true }) call ensures focus state is cleared synchronously before updating the index, preventing the highlight-stuck issue. The removal of the hoverEnabled re-enabling logic eliminates a problematic state toggle that caused event listener cleanup issues. Both fixes address real bugs without changing the overall architecture or adding complexity.
• No files require special attention

Important Files Changed

Filename	Overview
packages/react/src/floating-ui-react/hooks/useListNavigation.ts	Added synchronous focus clearing to fix race condition where onFocus fires after onPointerLeave with null
packages/react/src/menu/positioner/MenuPositioner.tsx	Removed logic that re-enabled hover on parent when child closes to prevent race conditions

[michaldudak]

Any particular reason for this change?

[atomiks]

No, was just testing things. I left it because I prefer the formatting