[mdm317]

• I have followed (at least) the PR section of the contributing guide.

Problem

When the pointer moves from one item to another, pointerleave's relatedTarget is the element it lands on:

<div role="option">   <!-- Item 1 -->
  <span>Apple</span>
</div>
<div role="option">   <!-- Item 2: landing on its padding works -->
  <span>Banana</span> <!-- landing here directly broke the check -->
</div>

• Landing on Item 2's own box (for example its padding): relatedTarget is Item 2, so the highlight was kept.
• Landing directly on a child of Item 2: relatedTarget is <span>Banana</span>.

onPointerLeave only checked listRef.current.includes(relatedTarget), so in the second case it treated the move as leaving the list and cleared the highlight. The next mousemove set it again, causing an extra state update and React commit.

Changes

Check whether relatedTarget is inside any item (with contains) instead of matching only the item elements, so child elements of items count too.

Side effect: consistent highlight while scrolling

Hovering an item and then scrolling the list without moving the pointer:

• Before: the highlight was sometimes cleared and sometimes kept.
• After: the highlight is always kept.

Performance

Measured in headless Chromium with a production React 19 build and 4x CPU throttling, as medians of 30 interleaved runs. Each item's text fills the item, so the pointer enters the next item's text directly. Each run parks the pointer on Item 0, then moves it with real mouse input one item at a time to Item 20 (20 boundary crossings).

Select

Scenario (20 crossings)	Items	Before	After	Delta
React commits	21	40	20	−50%
Script time	21	73.2 ms	55.9 ms	−24%
Main-thread task time	21	111.5 ms	90.3 ms	−19%
React commits	100	40	20	−50%
Script time	100	85.3 ms	58.7 ms	−31%
Main-thread task time	100	128.8 ms	98.5 ms	−24%

Combobox (100 items)

Scenario (20 crossings)	Before	After	Delta
React commits	80	40	−50%
Script time	92.1 ms	63.5 ms	−31%
Main-thread task time	128.5 ms	95.3 ms	−26%

Tests

Added to popupListConformanceTests

[pkg-pr-new]

• base-ui-tanstack-start

• vite-css-base-ui-example

  pnpm add https://pkg.pr.new/mui/base-ui/@base-ui/react@6199
  

  pnpm add https://pkg.pr.new/mui/base-ui/@base-ui/utils@6199
  

commit: 9d1a650

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[code-infra-dashboard]

Bundle size

Bundle	Parsed size	Gzip size
@base-ui/react	🔺+32B(+0.01%)	🔺+3B(0.00%)

Details of bundle changes

Performance

No regressions in 36 benchmarks · ran 4m 11s · details

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