React 19: patch to support legacy inert attribute values

[jsnajdr]

This is another patch we're applying to React 19 to improve compat with React 18. When faced with a <div inert=""> element, both versions treat it differently. React 18 sets the empty string value as-is as the DOM attribute value. And in DOM, any value, even '', means "true". React 19, however, converts the empty string to boolean, and because empty string is falsy, the result is "false".

We're applying a patch that interprets the '' value universally as true. In react-dom there is code:

case 'inert': {
  if (__DEV__) {
    if (value === '' && !didWarnForNewBooleanPropsWithEmptyValue[key]) {
      didWarnForNewBooleanPropsWithEmptyValue[key] = true;
      console.error(
        'Received an empty string for a boolean attribute `%s`. ' +
          'This will treat the attribute as if it were false. ' +
          'Either pass `false` to silence this warning, or ' +
          'pass `true` if you used an empty string in earlier versions of React to indicate this attribute is true.',
        key,
      );
    }
  }
}
// Fallthrough for boolean props that don't have a warning for empty strings.

which issues a warning in dev mode for this tricky case, but otherwise does nothing. Our patch adds this statement at the end of the case statement:

if ( "string" === typeof value ) value = true;

And that coerces the string value to true.

I adapted the e2e tests that checks for these incompatibilities: the previous check for inert wasn't catching the problematic case.

The PR also does a refactoring of the patching code, as it has grown somewhat since it was initially written.

[github-actions]

Size Change: +25 B (0%)

Total Size: 7.51 MB

📦 View Changed

Filename	Size	Change
build/scripts/vendors/react-dom-19.min.js	58.5 kB	+25 B (+0.04%)

_{compressed-size-action}

[jsnajdr]

By the way, this patch again corresponds to a feature flag that used to be there during the React 18 -> 19 transition. It was called enableNewBooleanProps and was cleaned up in react/react#28676.

[github-actions]

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[github-actions]

Flaky tests detected in ff4c586.
Some tests passed with failed attempts. The failures may not be related to this commit but are still reported for visibility. See the documentation for more information.

🔍 Workflow run URL: https://github.com/WordPress/gutenberg/actions/runs/28085515159
📝 Reported issues:

• [Flaky Test] Should save the changes #78689 in /test/e2e/specs/editor/plugins/wp-editor-meta-box.spec.js