Iframe: calc compat styles once per page load

packages/block-editor/src/components/iframe/get-compatibility-styles.js

@@ -0,0 +1,120 @@
+let compatibilityStyles = null;
+
+/**
+ * Returns a list of stylesheets that target the editor canvas. A stylesheet is
+ * considered targetting the editor a canvas if it contains the
+ * `editor-styles-wrapper`, `wp-block`, or `wp-block-*` class selectors.
+ *
+ * Ideally, this hook should be removed in the future and styles should be added
+ * explicitly as editor styles.
+ */
+export function getCompatibilityStyles() {
+	if ( compatibilityStyles ) {
+		return compatibilityStyles;
+	}
+
+	// Only memoize the result once on load, since these stylesheets should not
+	// change.
+	compatibilityStyles = Array.from( document.styleSheets ).reduce(
+		( accumulator, styleSheet ) => {
+			try {
+				// May fail for external styles.
+				// eslint-disable-next-line no-unused-expressions
+				styleSheet.cssRules;
+			} catch ( e ) {
+				return accumulator;
+			}
+
+			const { ownerNode, cssRules } = styleSheet;
+
+			// Stylesheet is added by another stylesheet. See
+			// https://developer.mozilla.org/en-US/docs/Web/API/StyleSheet/ownerNode#notes.
+			if ( ownerNode === null ) {
+				return accumulator;
+			}
+
+			if ( ! cssRules ) {
+				return accumulator;
+			}
+
+			// Don't try to add the reset styles, which were removed as a dependency
+			// from `edit-blocks` for the iframe since we don't need to reset admin
+			// styles.
+			if ( ownerNode.id === 'wp-reset-editor-styles-css' ) {
+				return accumulator;
+			}
+
+			// Don't try to add styles without ID. Styles enqueued via the WP dependency system will always have IDs.
+			if ( ! ownerNode.id ) {
+				return accumulator;
+			}
+
+			function matchFromRules( _cssRules ) {
+				return Array.from( _cssRules ).find(
+					( {
+						selectorText,
+						conditionText,
+						cssRules: __cssRules,
+					} ) => {
+						// If the rule is conditional then it will not have selector text.
+						// Recurse into child CSS ruleset to determine selector eligibility.
+						if ( conditionText ) {
+							return matchFromRules( __cssRules );
+						}
+
+						return (
+							selectorText &&
+							( selectorText.includes(
+								'.editor-styles-wrapper'
+							) ||
+								selectorText.includes( '.wp-block' ) )
+						);
+					}
+				);
+			}
+
+			if ( matchFromRules( cssRules ) ) {
+				const isInline = ownerNode.tagName === 'STYLE';
+
+				if ( isInline ) {
+					// If the current target is inline,
+					// it could be a dependency of an existing stylesheet.
+					// Look for that dependency and add it BEFORE the current target.
+					const mainStylesCssId = ownerNode.id.replace(
+						'-inline-css',
+						'-css'
+					);
+					const mainStylesElement =
+						document.getElementById( mainStylesCssId );
+					if ( mainStylesElement ) {
+						accumulator.push( mainStylesElement.cloneNode( true ) );
+					}
+				}
+
+				accumulator.push( ownerNode.cloneNode( true ) );
+
+				if ( ! isInline ) {
+					// If the current target is not inline,
+					// we still look for inline styles that could be relevant for the current target.
+					// If they exist, add them AFTER the current target.
+					const inlineStylesCssId = ownerNode.id.replace(
+						'-css',
+						'-inline-css'
+					);
+					const inlineStylesElement =
+						document.getElementById( inlineStylesCssId );
+					if ( inlineStylesElement ) {
+						accumulator.push(
+							inlineStylesElement.cloneNode( true )
+						);
+					}
+				}
+			}
+
+			return accumulator;
+		},
+		[]
+	);
+
+	return compatibilityStyles;
+}

packages/block-editor/src/components/iframe/index.js

@@ -28,7 +28,7 @@ import { useSelect } from '@wordpress/data';
  */
 import { useBlockSelectionClearer } from '../block-selection-clearer';
 import { useWritingFlow } from '../writing-flow';
-import { useCompatibilityStyles } from './use-compatibility-styles';
+import { getCompatibilityStyles } from './get-compatibility-styles';
 import { store as blockEditorStore } from '../../store';
 
 function bubbleEvent( event, Constructor, frame ) {
@@ -121,7 +121,6 @@ function Iframe( {
 	const { styles = '', scripts = '' } = resolvedAssets;
 	const [ iframeDocument, setIframeDocument ] = useState();
 	const [ bodyClasses, setBodyClasses ] = useState( [] );
-	const compatStyles = useCompatibilityStyles();
 	const clearerRef = useBlockSelectionClearer();
 	const [ before, writingFlowRef, after ] = useWritingFlow();
 	const [ contentResizeListener, { height: contentHeight } ] =
@@ -156,7 +155,7 @@ function Iframe( {
 
 			contentDocument.dir = ownerDocument.dir;
 
-			for ( const compatStyle of compatStyles ) {
+			for ( const compatStyle of getCompatibilityStyles() ) {
 				if ( contentDocument.getElementById( compatStyle.id ) ) {
 					continue;
 				}