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diff --git a/scripts-greasemonkey/gm4-polyfill.js b/scripts-greasemonkey/gm4-polyfill.js
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+++ b/scripts-greasemonkey/gm4-polyfill.js
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+/*
+This helper script bridges compatibility between the Greasemonkey 4 APIs and
+existing/legacy APIs. Say for example your user script includes
+
+    // @grant GM_getValue
+
+And you'd like to be compatible with both Greasemonkey 3 and Greasemonkey 4
+(and for that matter all versions of Violentmonkey, Tampermonkey, and any other
+user script engine).  Add:
+
+    // @grant GM.getValue
+    // @require https://greasemonkey.github.io/gm4-polyfill/gm4-polyfill.js
+
+And switch to the new (GM-dot) APIs, which return promises.  If your script
+is running in an engine that does not provide the new asynchronous APIs, this
+helper will add them, based on the old APIs.
+
+If you use `await` at the top level, you'll need to wrap your script in an
+`async` function to be compatible with any user script engine besides
+Greasemonkey 4.
+
+    (async () => {
+    let x = await GM.getValue('x');
+    })();
+*/
+
+if (typeof GM == 'undefined') {
+  this.GM = {};
+}
+
+
+if (typeof GM_addStyle == 'undefined') {
+  this.GM_addStyle = (aCss) => {
+    'use strict';
+    let head = document.getElementsByTagName('head')[0];
+    if (head) {
+      let style = document.createElement('style');
+      style.setAttribute('type', 'text/css');
+      style.textContent = aCss;
+      head.appendChild(style);
+      return style;
+    }
+    return null;
+  };
+}
+
+
+if (typeof GM_registerMenuCommand == 'undefined') {
+  this.GM_registerMenuCommand = (caption, commandFunc, accessKey) => {
+    if (!document.body) {
+      if (document.readyState === 'loading'
+        && document.documentElement && document.documentElement.localName === 'html') {
+        new MutationObserver((mutations, observer) => {
+          if (document.body) {
+            observer.disconnect();
+            GM_registerMenuCommand(caption, commandFunc, accessKey);
+          }
+        }).observe(document.documentElement, {childList: true});
+      } else {
+        console.error('GM_registerMenuCommand got no body.');
+      }
+      return;
+    }
+    let contextMenu = document.body.getAttribute('contextmenu');
+    let menu = (contextMenu ? document.querySelector('menu#' + contextMenu) : null);
+    if (!menu) {
+      menu = document.createElement('menu');
+      menu.setAttribute('id', 'gm-registered-menu');
+      menu.setAttribute('type', 'context');
+      document.body.appendChild(menu);
+      document.body.setAttribute('contextmenu', 'gm-registered-menu');
+    }
+    let menuItem = document.createElement('menuitem');
+    menuItem.textContent = caption;
+    menuItem.addEventListener('click', commandFunc, true);
+    menu.appendChild(menuItem);
+  };
+}
+
+
+if (typeof GM_getResourceText == 'undefined') {
+  this.GM_getResourceText = (aRes) => {
+    'use strict';
+    return GM.getResourceUrl(aRes)
+      .then(url => fetch(url))
+      .then(resp => resp.text())
+      .catch(function(error) {
+        GM.log('Request failed', error);
+        return null;
+      });
+  };
+}
+
+
+Object.entries({
+  'log': console.log.bind(console),  // Pale Moon compatibility.  See #13.
+  'info': GM_info,
+}).forEach(([newKey, old]) => {
+  if (old && (typeof GM[newKey] == 'undefined')) {
+    GM[newKey] = old;
+  }
+});
+
+
+Object.entries({
+  'GM_addStyle': 'addStyle',
+  'GM_deleteValue': 'deleteValue',
+  'GM_getResourceURL': 'getResourceUrl',
+  'GM_getValue': 'getValue',
+  'GM_listValues': 'listValues',
+  'GM_notification': 'notification',
+  'GM_openInTab': 'openInTab',
+  'GM_registerMenuCommand': 'registerMenuCommand',
+  'GM_setClipboard': 'setClipboard',
+  'GM_setValue': 'setValue',
+  'GM_xmlhttpRequest': 'xmlHttpRequest',
+  'GM_getResourceText': 'getResourceText',
+}).forEach(([oldKey, newKey]) => {
+  let old = this[oldKey];
+  if (old && (typeof GM[newKey] == 'undefined')) {
+    GM[newKey] = function(...args) {
+      return new Promise((resolve, reject) => {
+        try {
+          resolve(old.apply(this, args));
+        } catch (e) {
+          reject(e);
+        }
+      });
+    };
+  }
+});