Bug 1209970 - Fire scroll events early in the refresh tick. r=mats

With APZ we want to be firing scroll events to content more consistently, so
we tie them to the refresh driver tick rather than firing them on paint or
haphazardly on the next spin of the event loop.

Patch by Markus Stange, test fixes by Kartikaya Gupta

browser/components/sessionstore/test/browser_scrollPositions.js

@@ -16,6 +16,8 @@ const SCROLL2_X = Math.round(300 * (1 + Math.random()));
 const SCROLL2_Y = Math.round(400 * (1 + Math.random()));
 const SCROLL2_STR = SCROLL2_X + "," + SCROLL2_Y;
 
+requestLongerTimeout(2);
+
 /**
  * This test ensures that we properly serialize and restore scroll positions
  * for an average page without any frames.

layout/generic/nsGfxScrollFrame.cpp

@@ -4225,18 +4225,28 @@ void ScrollFrameHelper::CurPosAttributeChanged(nsIContent* aContent)
 
 /* ============= Scroll events ========== */
 
-NS_IMETHODIMP
-ScrollFrameHelper::ScrollEvent::Run()
+ScrollFrameHelper::ScrollEvent::ScrollEvent(ScrollFrameHelper* aHelper)
+  : mHelper(aHelper)
 {
-  if (mHelper)
-    mHelper->FireScrollEvent();
-  return NS_OK;
+  mHelper->mOuter->PresContext()->RefreshDriver()->AddRefreshObserver(this, Flush_Style);
+}
+
+ScrollFrameHelper::ScrollEvent::~ScrollEvent()
+{
+  mHelper->mOuter->PresContext()->RefreshDriver()->RemoveRefreshObserver(this, Flush_Style);
+}
+
+void
+ScrollFrameHelper::ScrollEvent::WillRefresh(mozilla::TimeStamp aTime)
+{
+  mHelper->FireScrollEvent();
 }
 
 void
 ScrollFrameHelper::FireScrollEvent()
 {
-  mScrollEvent.Forget();
+  MOZ_ASSERT(mScrollEvent);
+  mScrollEvent = nullptr;
 
   ActiveLayerTracker::SetCurrentScrollHandlerFrame(mOuter);
   WidgetGUIEvent event(true, eScroll, nullptr);
@@ -4263,14 +4273,11 @@ ScrollFrameHelper::FireScrollEvent()
 void
 ScrollFrameHelper::PostScrollEvent()
 {
-  if (mScrollEvent.IsPending())
+  if (mScrollEvent)
     return;
 
-  nsRootPresContext* rpc = mOuter->PresContext()->GetRootPresContext();
-  if (!rpc)
-    return;
+  // The ScrollEvent constructor registers itself with the refresh driver.
   mScrollEvent = new ScrollEvent(this);
-  rpc->AddWillPaintObserver(mScrollEvent.get());
 }
 
 NS_IMETHODIMP

layout/generic/nsGfxScrollFrame.h

@@ -19,6 +19,7 @@
 #include "nsIReflowCallback.h"
 #include "nsBoxLayoutState.h"
 #include "nsQueryFrame.h"
+#include "nsRefreshDriver.h"
 #include "nsExpirationTracker.h"
 #include "TextOverflow.h"
 #include "ScrollVelocityQueue.h"
@@ -101,11 +102,13 @@ public:
 
   bool IsSmoothScrollingEnabled();
 
-  class ScrollEvent : public nsRunnable {
+  class ScrollEvent : public nsARefreshObserver {
   public:
-    NS_DECL_NSIRUNNABLE
-    explicit ScrollEvent(ScrollFrameHelper *helper) : mHelper(helper) {}
-    void Revoke() { mHelper = nullptr; }
+    NS_INLINE_DECL_REFCOUNTING(ScrollEvent, override)
+    explicit ScrollEvent(ScrollFrameHelper *helper);
+    void WillRefresh(mozilla::TimeStamp aTime) override;
+  protected:
+    virtual ~ScrollEvent();
   private:
     ScrollFrameHelper *mHelper;
   };
@@ -417,7 +420,7 @@ public:
   nsCOMPtr<nsIContent> mScrollCornerContent;
   nsCOMPtr<nsIContent> mResizerContent;
 
-  nsRevocableEventPtr<ScrollEvent> mScrollEvent;
+  RefPtr<ScrollEvent> mScrollEvent;
   nsRevocableEventPtr<AsyncScrollPortEvent> mAsyncScrollPortEvent;
   nsRevocableEventPtr<ScrolledAreaEvent> mScrolledAreaEvent;
   nsIFrame* mHScrollbarBox;

testing/mochitest/tests/SimpleTest/EventUtils.js

@@ -526,8 +526,10 @@ function sendWheelAndPaint(aTarget, aOffsetX, aOffsetY, aEvent, aCallback, aWind
     setTimeout(function() {
       utils.advanceTimeAndRefresh(1000);
 
-      if (!aCallback)
+      if (!aCallback) {
+        utils.advanceTimeAndRefresh(0);
         return;
+      }
 
       var waitForPaints = function () {
         SpecialPowers.Services.obs.removeObserver(waitForPaints, "apz-repaints-flushed", false);

widget/tests/window_wheeltransaction.xul

@@ -1034,6 +1034,7 @@ function initElements()
   resetScrollPosition(gSubView1);
   resetScrollPosition(gSubView2);
   resetScrollPosition(gSubView3);
+  _getDOMWindowUtils(window).advanceTimeAndRefresh(0);
 
   runNextTestStep();
 }