Issue 4253 - Implement a less fragile way to detect documents that element hiding needs to apply to

lib/child/elemHide.js

Index: lib/child/elemHide.js
===================================================================
--- a/lib/child/elemHide.js
+++ b/lib/child/elemHide.js
@@ -297,17 +297,17 @@ HitRegistrationChannel.prototype = {
   }
 };
 
 let observer = {
   QueryInterface: XPCOMUtils.generateQI([
     Ci.nsIObserver, Ci.nsISupportsWeakReference
   ]),
 
-  topic: "content-document-global-created",
+  topic: "document-element-inserted",
   styleURL: Utils.makeURI("about:abp-elemhide?css"),
   sheet: null,
 
   init: function()
   {
     Services.obs.addObserver(this, this.topic, true);
     onShutdown.add(() =>
     {
@@ -320,72 +320,69 @@ let observer = {
     });
   },
 
   observe: function(subject, topic, data)
   {
     if (topic != this.topic)
       return;
 
-    if (subject.document.readyState == "uninitialized")
+    let window = subject.defaultView;
+    if (!window)
     {
-      // It would be nice to listen to the readystatechange event here. However,
-      // adding event listeners on uninitialized documents isn't possible. So
-      // we listen for DOMContentLoaded and beforescriptexecute - whichever
-      // fires first.
-      let listener = () =>
-      {
-        subject.removeEventListener("DOMContentLoaded", listener);
-        subject.removeEventListener("beforescriptexecute", listener);
-        this.observe(subject, topic, data);
-      };
-      subject.addEventListener("DOMContentLoaded", listener);
-      subject.addEventListener("beforescriptexecute", listener);
+      // This is typically XBL bindings and SVG images, but also real
+      // documents occasionally - probably due to speculative loading?
       return;
     }
+    let type = window.QueryInterface(Ci.nsIInterfaceRequestor)
+                     .getInterface(Ci.nsIWebNavigation)
+                     .QueryInterface(Ci.nsIDocShellTreeItem)
+                     .itemType;
+    if (type != Ci.nsIDocShellTreeItem.typeContent)
+      return;
 
     port.emitWithResponse("elemhideEnabled", {
-      frames: getFrames(subject),
-      isPrivate: isPrivate(subject)
+      frames: getFrames(window),
+      isPrivate: isPrivate(window)
     }).then(({
       enabled, contentType, docDomain, thirdParty, location, filter,
       filterType
     }) =>
     {
-      if (Cu.isDeadWrapper(subject))
+      if (Cu.isDeadWrapper(window))
       {
         // We are too late, the window is gone already.
         return;
       }
 
       if (enabled)
       {
         if (!this.sheet)
         {
           this.sheet = Utils.styleService.preloadSheet(this.styleURL,
               Ci.nsIStyleSheetService.USER_SHEET);
         }
 
-        let utils = subject.QueryInterface(Ci.nsIInterfaceRequestor)
-                           .getInterface(Ci.nsIDOMWindowUtils);
+        let utils = window.QueryInterface(Ci.nsIInterfaceRequestor)
+                          .getInterface(Ci.nsIDOMWindowUtils);
         try
         {
           utils.addSheet(this.sheet, Ci.nsIStyleSheetService.USER_SHEET);
         }
         catch (e)
         {
           // Ignore NS_ERROR_ILLEGAL_VALUE - it will be thrown if we try to add
           // the stylesheet multiple times to the same document (the observer
           // will be notified twice for some documents).
           if (e.result != Cr.NS_ERROR_ILLEGAL_VALUE)
             throw e;
         }
       }
       else if (filter)
       {
-        RequestNotifier.addNodeData(subject.document, subject.top, {
+        RequestNotifier.addNodeData(window.document, window.top, {
           contentType, docDomain, thirdParty, location, filter, filterType
         });
       }
     });
   }
 };
 observer.init();