Bug 696248. Flush pending onscroll events before painting. r=mats

layout/base/nsPresContext.cpp

@@ -2700,3 +2700,27 @@ nsRootPresContext::EnsureEventualDidPaintEvent()
   mNotifyDidPaintTimer->InitWithFuncCallback(NotifyDidPaintForSubtreeCallback,
                                              (void*)this, 100, nsITimer::TYPE_ONE_SHOT);
 }
+
+void
+nsRootPresContext::AddWillPaintObserver(nsIRunnable* aRunnable)
+{
+  if (!mWillPaintFallbackEvent.IsPending()) {
+    mWillPaintFallbackEvent = new RunWillPaintObservers(this);
+    NS_DispatchToMainThread(mWillPaintFallbackEvent.get());
+  }
+  mWillPaintObservers.AppendElement(aRunnable);
+}
+
+/**
+ * Run all runnables that need to get called before the next paint.
+ */
+void
+nsRootPresContext::FlushWillPaintObservers()
+{
+  mWillPaintFallbackEvent = nsnull;
+  nsTArray<nsCOMPtr<nsIRunnable> > observers;
+  observers.SwapElements(mWillPaintObservers);
+  for (PRUint32 i = 0; i < observers.Length(); ++i) {
+    observers[i]->Run();
+  }
+}

layout/base/nsPresContext.h

@@ -1304,12 +1304,40 @@ public:
    */
   PRUint32 GetDOMGeneration() { return mDOMGeneration; }
 
-private:
+  /**
+   * Add a runnable that will get called before the next paint. They will get
+   * run eventually even if painting doesn't happen. They might run well before
+   * painting happens.
+   */
+  void AddWillPaintObserver(nsIRunnable* aRunnable);
+
+  /**
+   * Run all runnables that need to get called before the next paint.
+   */
+  void FlushWillPaintObservers();
+
+protected:
+  class RunWillPaintObservers : public nsRunnable {
+  public:
+    RunWillPaintObservers(nsRootPresContext* aPresContext) : mPresContext(aPresContext) {}
+    void Revoke() { mPresContext = nsnull; }
+    NS_IMETHOD Run()
+    {
+      if (mPresContext) {
+        mPresContext->FlushWillPaintObservers();
+      }
+      return NS_OK;
+    }
+    nsRootPresContext* mPresContext;
+  };
+
   nsCOMPtr<nsITimer> mNotifyDidPaintTimer;
   nsTHashtable<nsPtrHashKey<nsObjectFrame> > mRegisteredPlugins;
   // if mNeedsToUpdatePluginGeometry is set, then this is the frame to
   // use as the root of the subtree to search for plugin updates, or
   // null to use the root frame of this prescontext
+  nsTArray<nsCOMPtr<nsIRunnable> > mWillPaintObservers;
+  nsRevocableEventPtr<RunWillPaintObservers> mWillPaintFallbackEvent;
   nsIFrame* mUpdatePluginGeometryForFrame;
   PRUint32 mDOMGeneration;
   bool mNeedsToUpdatePluginGeometry;

layout/base/nsPresShell.cpp

@@ -4460,6 +4460,13 @@ PresShell::RenderDocument(const nsRect& aRect, PRUint32 aFlags,
 
   NS_ENSURE_TRUE(!(aFlags & RENDER_IS_UNTRUSTED), NS_ERROR_NOT_IMPLEMENTED);
 
+  nsRootPresContext* rootPresContext = mPresContext->GetRootPresContext();
+  if (rootPresContext) {
+    rootPresContext->FlushWillPaintObservers();
+    if (mIsDestroying)
+      return NS_OK;
+  }
+
   nsAutoScriptBlocker blockScripts;
 
   // Set up the rectangle as the path in aThebesContext
@@ -6861,16 +6868,18 @@ PresShell::WillPaint(bool aWillSendDidPaint)
     return;
   }
 
-  if (!aWillSendDidPaint) {
-    nsRootPresContext* rootPresContext = mPresContext->GetRootPresContext();
-    if (!rootPresContext) {
-      return;
-    }
-    if (rootPresContext == mPresContext) {
-      rootPresContext->UpdatePluginGeometry();
-    }
+  nsRootPresContext* rootPresContext = mPresContext->GetRootPresContext();
+  if (!rootPresContext) {
+    return;
   }
 
+  if (!aWillSendDidPaint && rootPresContext == mPresContext) {
+    rootPresContext->UpdatePluginGeometry();
+  }
+  rootPresContext->FlushWillPaintObservers();
+  if (mIsDestroying)
+    return;
+
   // Process reflows, if we have them, to reduce flicker due to invalidates and
   // reflow being interspersed.  Note that we _do_ allow this to be
   // interruptible; if we can't do all the reflows it's better to flicker a bit

layout/generic/nsGfxScrollFrame.cpp

@@ -2643,12 +2643,11 @@ nsGfxScrollFrameInner::PostScrollEvent()
   if (mScrollEvent.IsPending())
     return;
 
-  nsRefPtr<ScrollEvent> ev = new ScrollEvent(this);
-  if (NS_FAILED(NS_DispatchToCurrentThread(ev))) {
-    NS_WARNING("failed to dispatch ScrollEvent");
-  } else {
-    mScrollEvent = ev;
-  }
+  nsRootPresContext* rpc = mOuter->PresContext()->GetRootPresContext();
+  if (!rpc)
+    return;
+  mScrollEvent = new ScrollEvent(this);
+  rpc->AddWillPaintObserver(mScrollEvent.get());
 }
 
 NS_IMETHODIMP