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diff --git a/data/extensions/uBlock0@raymondhill.net/js/resources/run-at.js b/data/extensions/uBlock0@raymondhill.net/js/resources/run-at.js
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+++ b/data/extensions/uBlock0@raymondhill.net/js/resources/run-at.js
@@ -0,0 +1,96 @@
+/*******************************************************************************
+
+    uBlock Origin - a comprehensive, efficient content blocker
+    Copyright (C) 2019-present Raymond Hill
+
+    This program is free software: you can redistribute it and/or modify
+    it under the terms of the GNU General Public License as published by
+    the Free Software Foundation, either version 3 of the License, or
+    (at your option) any later version.
+
+    This program is distributed in the hope that it will be useful,
+    but WITHOUT ANY WARRANTY; without even the implied warranty of
+    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+    GNU General Public License for more details.
+
+    You should have received a copy of the GNU General Public License
+    along with this program.  If not, see {http://www.gnu.org/licenses/}.
+
+    Home: https://github.com/gorhill/uBlock
+
+*/
+
+import { registerScriptlet } from './base.js';
+import { safeSelf } from './safe-self.js';
+
+/* eslint no-prototype-builtins: 0 */
+
+/**
+ * @helperScriptlet run-at.fn
+ * 
+ * @description
+ * Execute a function at a specific page-load milestone.
+ * 
+ * @param fn
+ * The function to call.
+ * 
+ * @param when
+ * An identifier which tells when the function should be executed.
+ * See <https://developer.mozilla.org/en-US/docs/Web/API/Document/readyState>.
+ * 
+ * @example
+ * `runAt(( ) => { start(); }, 'interactive')`
+ * 
+ * */
+
+export function runAt(fn, when) {
+    const intFromReadyState = state => {
+        const targets = {
+            'loading': 1, 'asap': 1,
+            'interactive': 2, 'end': 2, '2': 2,
+            'complete': 3, 'idle': 3, '3': 3,
+        };
+        const tokens = Array.isArray(state) ? state : [ state ];
+        for ( const token of tokens ) {
+            const prop = `${token}`;
+            if ( Object.hasOwn(targets, prop) === false ) { continue; }
+            return targets[prop];
+        }
+        return 0;
+    };
+    const runAt = intFromReadyState(when);
+    if ( intFromReadyState(document.readyState) >= runAt ) {
+        fn(); return;
+    }
+    const onStateChange = ( ) => {
+        if ( intFromReadyState(document.readyState) < runAt ) { return; }
+        fn();
+        safe.removeEventListener.apply(document, args);
+    };
+    const safe = safeSelf();
+    const args = [ 'readystatechange', onStateChange, { capture: true } ];
+    safe.addEventListener.apply(document, args);
+}
+registerScriptlet(runAt, {
+    name: 'run-at.fn',
+    dependencies: [
+        safeSelf,
+    ],
+});
+
+/******************************************************************************/
+
+export function runAtHtmlElementFn(fn) {
+    if ( document.documentElement ) {
+        fn();
+        return;
+    }
+    const observer = new MutationObserver(( ) => {
+        observer.disconnect();
+        fn();
+    });
+    observer.observe(document, { childList: true });
+}
+registerScriptlet(runAtHtmlElementFn, {
+    name: 'run-at-html-element.fn',
+});