Issue 6113 - Improve reliability of "block element" dialog

composer.js

 /*
  * This file is part of Adblock Plus <https://adblockplus.org/>,
  * Copyright (C) 2006-present eyeo GmbH
  *
  * Adblock Plus is free software: you can redistribute it and/or modify
  * it under the terms of the GNU General Public License version 3 as
  * published by the Free Software Foundation.
  *
  * Adblock Plus is distributed in the hope that it will be useful,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
@@ skipping 82 matching lines @@
   ext.onMessage.addListener((msg, sender, sendResponse) =>
   {
     switch (msg.type)
     {
       case "composer.dialog.init":
         targetPageId = msg.sender;
         let filtersTextArea = document.getElementById("filters");
         filtersTextArea.value = msg.filters.join("\n");
         filtersTextArea.disabled = false;
         $("#addButton").button("option", "disabled", false);
+
+        // Firefox sometimes tells us this window had loaded before it has[1],
+        // to work around that we send the "composer.dialog.init" message again
+        // when sending failed. Unfortunately sometimes sending is reported as
+        // successful when it's not, but with the response of `undefined`. We
+        // therefore send a response here, and check for it to see if the
+        // message really was sent successfully.
+        // [1] - https://bugzilla.mozilla.org/show_bug.cgi?id=1418655
+        sendResponse(true);
         break;
       case "composer.dialog.close":
         closeMe();
         break;
     }
   });
 
   window.removeEventListener("load", init);
 }
 window.addEventListener("load", init, false);