Bug 870101 - Make BackgroundPageThumbs load about:blank after capturing windows so that they're eventually collected. r=markh

toolkit/components/thumbnails/content/backgroundPageThumbsContent.js

@@ -21,9 +21,6 @@ const backgroundPageThumbsContent = {
     docShell.allowMedia = false;
     docShell.allowPlugins = false;
 
-    // Stop about:blank from loading.  If it finishes loading after a capture
-    // request is received, it could trigger the capture's load listener.
-    this._webNav.stop(Ci.nsIWebNavigation.STOP_NETWORK);
     addMessageListener("BackgroundPageThumbs:capture",
                        this._onCapture.bind(this));
   },
@@ -33,10 +30,9 @@ const backgroundPageThumbsContent = {
   },
 
   _onCapture: function (msg) {
-    if (this._onLoad) {
-      this._webNav.stop(Ci.nsIWebNavigation.STOP_NETWORK);
+    this._webNav.stop(Ci.nsIWebNavigation.STOP_NETWORK);
+    if (this._onLoad)
       removeEventListener("load", this._onLoad, true);
-    }
 
     this._onLoad = function onLoad(event) {
       if (event.target != content.document)
@@ -58,6 +54,11 @@ const backgroundPageThumbsContent = {
         });
       };
       canvas.toBlob(blob => fileReader.readAsArrayBuffer(blob));
+
+      // Load about:blank to cause the captured window to be collected...
+      // eventually.
+      this._webNav.loadURI("about:blank", Ci.nsIWebNavigation.LOAD_FLAGS_NONE,
+                           null, null, null);
     }.bind(this);
 
     addEventListener("load", this._onLoad, true);