Add ability to linger for remove-class scriptlet

Similar to related change for the `remove-attr` scriptlet:
- 0f330c7359

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

assets/resources/scriptlets.js

@@ -764,7 +764,10 @@
     if ( selector === '' || selector === '{{2}}' ) {
         selector = '.' + tokens.map(a => CSS.escape(a)).join(',.');
     }
+    let behavior = '{{3}}';
+    let timer;
     const rmclass = function() {
+        timer = undefined;
         try {
             const nodes = document.querySelectorAll(selector);
             for ( const node of nodes ) {
@@ -773,14 +776,39 @@
         } catch(ex) {
         }
     };
-    if ( document.readyState === 'loading' ) {
-        window.addEventListener(
-            'DOMContentLoaded',
-            rmclass,
-            { capture: true, once: 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();
     }
 })();