Bug: Removal of custom element property sets it to `null` rather than `undefined`

[augustjk]

React version: experimental

Steps To Reproduce

1. Set up a React project with react@experimental & react-dom@experimental which includes custom element property support.
2. Render a custom element with a property value provided as prop, then on a re-render remove that prop.

e.g.

function App() {
  const [condition, setCondition] = useState(false);
  return (
    <>
      <button onClick={() => setCondition(c => !c)}>toggle</button>
      {condition ? <my-element foo={"foo"} /> : <my-element />}
    </>
  );
}

Link to code example:
https://stackblitz.com/edit/vitejs-vite-dz2rka?file=src%2FApp.tsx

The current behavior

The value of the foo property on <my-element> will start off undefined. Toggling condition to true will set the property to "foo", but subsequently toggling condition back to false will set the property to null.

The expected behavior

I would expect the property to be undefined.

[augustjk]

I'm guessing the source of this is behavior is here:

react/packages/react-dom-bindings/src/client/ReactDOMComponent.js

Lines 1776 to 1783 in 2efa383

	setPropOnCustomElement(
	domElement,
	tag,
	propKey,
	null,
	nextProps,
	lastProp,
	);

[augustjk]

@rickhanlonii I saw the update on #11347 about support coming in canary and next major. I just wanted to raise this particular issue so it might be looked at before custom element support gets a wider release.

[rickhanlonii]

Thanks for the ping @augustjk.

Confirmed that this is the behavior, we'll need to do some more investigating before deciding what to do here.

I tagged it as a React 19 issue though so we can address before stable.