[button] Fix custom keyboard click dispatch

[atomiks]

Custom-tag Buttons handled keyboard activation by directly calling the Button prop handler. This makes Enter and Space dispatch a real click instead, so render-element and delegated click handlers run through the normal event path.

Root cause

For non-native buttons, useButton called the captured onClick prop directly:

externalOnClick?.(event);

That skipped other click handlers on the rendered element, including cases like:

<Button nativeButton={false} render={<span onClick={handleClick} />} />

Changes

• Dispatch a real click for Enter and Space activation on non-native, non-composite buttons.
• Add missing public Button coverage around the affected keyboard path and nearby documented behavior: form submission, disabled state callbacks, and focusable disabled transitions.
• Keep the fix narrow and avoid broader render-prop disabled guard machinery for rare handler placement on disabled render elements.

[pkg-pr-new]

• base-ui-tanstack-start

• vite-css-base-ui-example

  pnpm add https://pkg.pr.new/mui/base-ui/@base-ui/react@4838
  

  pnpm add https://pkg.pr.new/mui/base-ui/@base-ui/utils@4838
  

commit: 5530a85

[code-infra-dashboard]

Bundle size

Bundle	Parsed size	Gzip size
@base-ui/react	🔺+74B(+0.02%)	🔺+14B(+0.01%)

Details of bundle changes

Performance

Total duration: 1,103.85 ms -116.18 ms^(-9.5%) | Renders: 50 ⁽⁺⁰⁾ | Paint: 1,694.30 ms -191.24 ms^(-10.1%)

No significant changes — details

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