Bug 1019999 - Clear overscroll if snap-back animation is cancelled. r=kats

--HG--
extra : source : 20ace5785615d1cc7d0edbe7e09f8691166c6166

gfx/layers/apz/src/AsyncPanZoomController.cpp

@@ -505,6 +505,16 @@ public:
     return continueX || continueY;
   }
 
+  virtual void Cancel() MOZ_OVERRIDE
+  {
+    // If the snap-back animation is cancelled for some reason, we need to
+    // clear the overscroll, otherwise the user would be stuck in the
+    // overscrolled state (since touch blocks beginning in an overscrolled
+    // state are ignored).
+    mApzc.mX.ClearOverscroll();
+    mApzc.mY.ClearOverscroll();
+  }
+
 private:
   AsyncPanZoomController& mApzc;
 };
@@ -1684,7 +1694,12 @@ void AsyncPanZoomController::StartAnimation(AsyncPanZoomAnimation* aAnimation)
 void AsyncPanZoomController::CancelAnimation() {
   ReentrantMonitorAutoEnter lock(mMonitor);
   SetState(NOTHING);
-  mAnimation = nullptr;
+  if (mAnimation) {
+    mAnimation->Cancel();
+    mAnimation = nullptr;
+    // mAnimation->Cancel() may have done something that requires a repaint.
+    RequestContentRepaint();
+  }
 }
 
 void AsyncPanZoomController::SetCompositorParent(CompositorParent* aCompositorParent) {

gfx/layers/apz/src/AsyncPanZoomController.h

@@ -1032,6 +1032,9 @@ public:
   virtual bool Sample(FrameMetrics& aFrameMetrics,
                       const TimeDuration& aDelta) = 0;
 
+  // Called if the animation is cancelled before it ends.
+  virtual void Cancel() {}
+
   /**
    * Get the deferred tasks in |mDeferredTasks|. See |mDeferredTasks|
    * for more information.

gfx/layers/apz/src/Axis.cpp

@@ -177,6 +177,10 @@ bool Axis::IsOverscrolled() const {
   return mOverscroll != 0;
 }
 
+void Axis::ClearOverscroll() {
+  mOverscroll = 0;
+}
+
 float Axis::PanDistance() {
   return fabsf(mPos - mStartPos);
 }

gfx/layers/apz/src/Axis.h

@@ -109,6 +109,11 @@ public:
    */
   bool IsOverscrolled() const;
 
+  /**
+   * Clear any overscroll amount on this axis.
+   */
+  void ClearOverscroll();
+
   /**
    * Gets the distance between the starting position of the touch supplied in
    * startTouch() and the current touch from the last