Bug 413778. Avoid creating and destroying extra empty nsDocAccessible when new tab opened. r=ginn.chen, a=blocking1.9

accessible/src/base/nsAccessibilityService.cpp

@@ -419,6 +419,19 @@ nsAccessibilityService::CreateRootAccessible(nsIPresShell *aShell,
   nsCOMPtr<nsIContentViewer> contentViewer;
   docShell->GetContentViewer(getter_AddRefs(contentViewer));
   NS_ENSURE_TRUE(contentViewer, NS_ERROR_FAILURE); // Doc was already shut down
+  PRUint32 busyFlags;
+  docShell->GetBusyFlags(&busyFlags);
+  if (busyFlags != nsIDocShell::BUSY_FLAGS_NONE) {
+    nsCOMPtr<nsIWebNavigation> webNav(do_GetInterface(docShell));
+    nsCOMPtr<nsIURI> uri;
+    webNav->GetCurrentURI(getter_AddRefs(uri));
+    NS_ENSURE_STATE(uri);
+    nsCAutoString url;
+    uri->GetSpec(url);
+    if (url.EqualsLiteral("about:blank")) {
+      return NS_OK;  // No load events for a busy about:blank -- they are often temporary
+    }
+  }
 
   nsCOMPtr<nsIDocShellTreeItem> docShellTreeItem =
     do_QueryInterface(container);