[mergeProps] Fix multi-argument event handler forwarding

[atomiks]

Fixes #4597

mergeProps only forwarded the first argument when merged handlers were called, so cases like mergeProps({ onOpenChange }, { onOpenChange }) were already dropping secondary arguments. PR #4330 added preventBaseUIHandler() runtime wrapping for lone and first-position handlers, which widened the same behavior to cases that had previously worked, such as mergeProps({}, { onOpenChange }).

This change forwards all handler arguments in both the merged-handler path and the runtime wrapper path, while still only treating the first argument as the synthetic event for preventBaseUIHandler().

Changes

• Forward all arguments from mergeEventHandlers().
• Forward all arguments from wrapEventHandler().
• Keep preventBaseUIHandler() behavior tied to the first argument.
• Add regression tests for merged callbacks, lone wrapped callbacks, and synthetic handlers with extra arguments.

[pkg-pr-new]

• base-ui-tanstack-start

• vite-css-base-ui-example

  pnpm add https://pkg.pr.new/mui/base-ui/@base-ui/react@4598
  

  pnpm add https://pkg.pr.new/mui/base-ui/@base-ui/utils@4598
  

commit: ba85b95

[code-infra-dashboard]

Bundle size

Bundle	Parsed size	Gzip size
@base-ui/react	🔺+54B(+0.01%)	🔺+13B(+0.01%)

Details of bundle changes

Performance

Total duration: 1,416.08 ms 🔺+59.39 ms^(+4.4%) | Renders: 53 ⁽⁺⁰⁾ | Paint: 2,172.02 ms +45.22 ms^(+2.1%)

Test	Duration	Renders
Select open (500 options)	57.76 ms 🔺+10.55 ms(+22.3%)	14 (+0)
Select mount (200 instances)	165.95 ms 🔺+27.17 ms(+19.6%)	3 (+0)
Menu open (500 items)	81.86 ms 🔺+10.99 ms(+15.5%)	12 (+0)
Scroll Area mount (300 instances)	98.82 ms 🔺+12.71 ms(+14.8%)	3 (+0)
Menu mount (300 instances)	153.46 ms 🔺+11.66 ms(+8.2%)	2 (+0)

...and 7 more. View full report

Details of benchmark changes

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[cgatian]

@atomiks verified locally 🚀 . Thank you!

[atomiks]

@cgatian surprisingly, it was already broken before (in a different way), as mergeProps({onOpenChange}, {onOpenChange}) already had this issue, while mergeProps({}, {onOpenChange}) worked

I'm not sure what the blast radius of the regression is like in terms of how commonly this was used, or if your case was somewhat unique

[cgatian]

@atomiks Hmm we were using it with what would seem like a pretty common use case

export const ConfirmDialogRoot: React.FC<ConfirmDialogRoot.Props> = (rest) => {
  const handleOnOpenChange: AlertDialog.Root.Props['onOpenChange'] = (_open, eventDetails) => {
     // code omitted for brevity
  };

  return (
      <AlertDialog.Root {...mergeProps(rest, { onOpenChange: handleOnOpenChange })} />
  );
};