Issue 4251 - Fixed hang trying to apply element hiding rules to blocked frames

lib/child/elemHide.js

Index: lib/child/elemHide.js
===================================================================
--- a/lib/child/elemHide.js
+++ b/lib/child/elemHide.js
@@ -323,22 +323,27 @@ let observer = {
   observe: function(subject, topic, data)
   {
     if (topic != this.topic)
       return;
 
     if (subject.document.readyState == "uninitialized")
     {
       // It would be nice to listen to the readystatechange event here. However,
-      // it doesn't fire when changing from "uninitialized" state.
-      Utils.runAsync(() =>
+      // adding event listeners on uninitialized documents isn't possible. So
+      // we listen for DOMContentLoaded and beforescriptexecute - whichever
+      // fires first.
+      let listener = () =>
       {
-        if (!Cu.isDeadWrapper(subject))
-          this.observe(subject, topic, data);
-      });
+        subject.removeEventListener("DOMContentLoaded", listener);
+        subject.removeEventListener("beforescriptexecute", listener);
+        this.observe(subject, topic, data);
+      };
+      subject.addEventListener("DOMContentLoaded", listener);
+      subject.addEventListener("beforescriptexecute", listener);

[Wladimir Palant, 2016/07/19 16:23:25]

For reference, DOMContentLoaded fires too late - with the test case from
https://issues.adblockplus.org/ticket/4211 I can see a noticeable delay (below
one second but still). In most cases, beforescriptexecute will fire way earlier
- especially in the scenarios that are relevant for us. With this additional
event listener I can no longer see the ads disappearing.

       return;
     }
 
     port.emitWithResponse("elemhideEnabled", {
       frames: getFrames(subject),
       isPrivate: isPrivate(subject)
     }).then(({
       enabled, contentType, docDomain, thirdParty, location, filter,