Bug 732564 - Add a before-first-paint event that fires before the first paint of the new document. r=Cwiiis, bz

layout/base/nsDocumentViewer.cpp

@@ -506,6 +506,18 @@ public:
   nsCOMPtr<nsIDocument> mTop;
 };
 
+class nsBeforeFirstPaintDispatcher : public nsRunnable
+{
+public:
+  nsBeforeFirstPaintDispatcher(nsIDocument* aDocument)
+  : mDocument(aDocument) {}
+
+  NS_IMETHOD Run();
+
+private:
+  nsCOMPtr<nsIDocument> mDocument;
+};
+
 class nsDocumentShownDispatcher : public nsRunnable
 {
 public:
@@ -2047,8 +2059,13 @@ DocumentViewerImpl::Show(void)
     }
   }
 
-  // Notify observers that a new page has been shown. (But not right now;
+  // Notify observers that a new page is about to be drawn. Execute this
-  // running JS at this time is not safe.)
+  // as soon as it is safe to run JS, which is guaranteed to be before we
+  // go back to the event loop and actually draw the page.
+  nsContentUtils::AddScriptRunner(new nsBeforeFirstPaintDispatcher(mDocument));
+
+  // Notify observers that a new page has been shown. This will get run
+  // from the event loop after we actually draw the page.
   NS_DispatchToMainThread(new nsDocumentShownDispatcher(mDocument));
 
   return NS_OK;
@@ -4370,9 +4387,21 @@ DocumentViewerImpl::SetPrintPreviewPresentation(nsIViewManager* aViewManager,
   mPresShell = aPresShell;
 }
 
-// Fires the "document-shown" event so that interested parties (right now, the
+// Fires the "before-first-paint" event so that interested parties (right now, the
 // mobile browser) are aware of it.
 NS_IMETHODIMP
+nsBeforeFirstPaintDispatcher::Run()
+{
+  nsCOMPtr<nsIObserverService> observerService =
+    mozilla::services::GetObserverService();
+  if (observerService) {
+    observerService->NotifyObservers(mDocument, "before-first-paint", NULL);
+  }
+  return NS_OK;
+}
+
+// Fires the "document-shown" event so that interested parties are aware of it.
+NS_IMETHODIMP
 nsDocumentShownDispatcher::Run()
 {
   nsCOMPtr<nsIObserverService> observerService =

mobile/android/chrome/content/browser.js

@@ -1517,7 +1517,7 @@ Tab.prototype = {
     this.browser.addEventListener("pagehide", this, true);
     this.browser.addEventListener("pageshow", this, true);
 
-    Services.obs.addObserver(this, "document-shown", false);
+    Services.obs.addObserver(this, "before-first-paint", false);
 
     if (!aParams.delayLoad) {
       let flags = "flags" in aParams ? aParams.flags : Ci.nsIWebNavigation.LOAD_FLAGS_NONE;
@@ -1563,7 +1563,7 @@ Tab.prototype = {
     this.browser.removeEventListener("pagehide", this, true);
     this.browser.removeEventListener("pageshow", this, true);
 
-    Services.obs.removeObserver(this, "document-shown");
+    Services.obs.removeObserver(this, "before-first-paint");
 
     // Make sure the previously selected panel remains selected. The selected panel of a deck is
     // not stable when panels are removed.
@@ -2238,7 +2238,7 @@ Tab.prototype = {
 
   observe: function(aSubject, aTopic, aData) {
     switch (aTopic) {
-      case "document-shown":
+      case "before-first-paint":
         // Is it on the top level?
         let contentDocument = aSubject;
         if (contentDocument == this.browser.contentDocument) {