Add ability to linger for remove-class scriptlet

Similar to related change for the `remove-attr` scriptlet:
- gorhill/uBlock@0f330c7

Related feedback:
- https://www.reddit.com/r/uBlockOrigin/comments/nsroaw/some_elements_isare_not_removed_after_the_cookie/

Related commits:
- gorhill/uBlock@2de24a1

Co-authored-by: Raymond Hill <rhill@raymondhill.net>

assets/resources/resources.txt

@@ -2276,28 +2276,58 @@ remove-attr.js application/javascript
 
 remove-class.js application/javascript
 (function() {
-	var token = '{{1}}';
+	const token = '{{1}}';
 	if ( token === '' || token === '{{1}}' ) { return; }
-	var tokens = token.split(/\s*\|\s*/);
-	var selector = '{{2}}';
+	const tokens = token.split(/\s*\|\s*/);
+	let selector = '{{2}}';
 	if ( selector === '' || selector === '{{2}}' ) {
 		selector = '.' + tokens.map(a => CSS.escape(a)).join(',.');
 	}
-	var rmclass = function(ev) {
-		if ( ev ) { window.removeEventListener(ev.type, rmclass, true); }
+	let behavior = '{{3}}';
+	let timer;
+	const rmclass = function() {
+		timer = undefined;
 		try {
-			var nodes = document.querySelectorAll(selector), i = nodes.length;
-			while ( i-- ) {
-				var node = nodes[i];
+			const nodes = document.querySelectorAll(selector);
+			for ( let node of nodes ) {
 				node.classList.remove(...tokens);
 			}
 		} catch(ex) {
 		}
 	};
-	if ( document.readyState === 'loading' ) {
-		window.addEventListener('DOMContentLoaded', rmclass, true);
-	} else {
+	const mutationHandler = mutations => {
+		if ( timer !== undefined ) { return; }
+		let skip = true;
+		for ( let i = 0; i < mutations.length && skip; i++ ) {
+			const { type, addedNodes, removedNodes } = mutations[i];
+			if ( type === 'attributes' ) { skip = false; }
+			for ( let j = 0; j < addedNodes.length && skip; j++ ) {
+				if ( addedNodes[j].nodeType === 1 ) { skip = false; break; }
+			}
+			for ( let j = 0; j < removedNodes.length && skip; j++ ) {
+				if ( removedNodes[j].nodeType === 1 ) { skip = false; break; }
+			}
+		}
+		if ( skip ) { return; }
+		timer = self.requestIdleCallback(rmclass, { timeout: 67 });
+	};
+	const start = ( ) => {
 		rmclass();
+		if ( /\bstay\b/.test(behavior) === false ) { return; }
+		const observer = new MutationObserver(mutationHandler);
+		observer.observe(document.documentElement, {
+			attributes: true,
+			attributeFilter: [ 'class' ],
+			childList: true,
+			subtree: true,
+		});
+	};
+	if ( document.readyState !== 'complete' && /\bcomplete\b/.test(behavior) ) {
+		self.addEventListener('load', start, { once: true });
+	} else if ( document.readyState === 'loading' ) {
+		self.addEventListener('DOMContentLoaded', start, { once: true });
+	} else {
+		start();
 	}
 })();