Issue 4101 - Prevent runaway MutationObservers

include.preload.js

 /*
  * This file is part of Adblock Plus <https://adblockplus.org/>,
  * Copyright (C) 2006-2016 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 132 matching lines @@
       type: "filters.collapse",
       urls: urls,
       mediatype: mediatype,
       baseURL: document.location.href
     },
 
     function(collapse)
     {
       function collapseElement()
       {
+        var propertyName = "display";
+        var propertyValue = "none";
         if (element.localName == "frame")
-          element.style.setProperty("visibility", "hidden", "important");
-        else
-          element.style.setProperty("display", "none", "important");
+        {
+          propertyName = "visibility";
+          propertyValue = "hidden";
+        }
+
+        if (element.style.getPropertyValue(propertyName) !== propertyValue ||
+            element.style.getPropertyPriority(propertyName) !== "important")
+          element.style.setProperty(propertyName, propertyValue, "important");
       }
 
       if (collapse && !element._collapsed)
       {
         collapseElement();
         element._collapsed = true;
 
         if (MutationObserver)
           new MutationObserver(collapseElement).observe(
             element, {
@@ skipping 441 matching lines @@
   }, true);
 
   return updateStylesheet;
 }
 
 if (document instanceof HTMLDocument)
 {
   checkSitekey();
   window.updateStylesheet = init(document);
 }