Bug 1040226 - Avoid getting stuck after overscroll is handed off to an APZC later in the handoff chain. r=kats

gfx/layers/apz/src/APZCTreeManager.cpp

@@ -907,6 +907,23 @@ APZCTreeManager::CancelAnimationsForOverscrollHandoffChain()
   return mOverscrollHandoffChain.length() > 0;
 }
 
+void
+APZCTreeManager::SnapBackOverscrolledApzc(AsyncPanZoomController* aApzc)
+{
+  // This is called when a fling is winding down, so there is no overscroll
+  // handoff chain already built, so build it now.
+  BuildOverscrollHandoffChain(aApzc);
+  MonitorAutoLock lock(mTreeLock);
+  // Exactly one APZC along the hand-off chain can be overscrolled.
+  // Find it, and start a snap-back animation for it.
+  for (uint32_t i = 0; i < mOverscrollHandoffChain.length(); ++i) {
+    if (mOverscrollHandoffChain[i]->SnapBackIfOverscrolled()) {
+      break;
+    }
+  }
+  mOverscrollHandoffChain.clear();
+}
+
 bool
 APZCTreeManager::CanBePanned(AsyncPanZoomController* aApzc)
 {

gfx/layers/apz/src/APZCTreeManager.h

@@ -294,6 +294,7 @@ public:
 
   bool FlushRepaintsForOverscrollHandoffChain();
   bool CancelAnimationsForOverscrollHandoffChain();
+  void SnapBackOverscrolledApzc(AsyncPanZoomController* aStart);
 
   /**
    * Determine whether |aApzc|, or any APZC along its overscroll handoff chain,

gfx/layers/apz/src/AsyncPanZoomController.cpp

@@ -496,10 +496,10 @@ public:
     // If we shouldn't continue the fling, let's just stop and repaint.
     if (!shouldContinueFlingX && !shouldContinueFlingY) {
       APZC_LOG("%p ending fling animation. overscrolled=%d\n", &mApzc, mApzc.IsOverscrolled());
-      // If we are in overscroll, schedule the snap-back animation that relieves it.
-      if (mApzc.IsOverscrolled()) {
-        mDeferredTasks.append(NewRunnableMethod(&mApzc, &AsyncPanZoomController::StartSnapBack));
-      }
+      // This APZC or an APZC further down the handoff chain may be be overscrolled.
+      // Start a snap-back animation on the overscrolled APZC.
+      mDeferredTasks.append(NewRunnableMethod(&mApzc,
+          &AsyncPanZoomController::CallSnapBackOverscrolledApzc));
       return false;
     }
 
@@ -654,6 +654,11 @@ public:
   OverscrollSnapBackAnimation(AsyncPanZoomController& aApzc)
     : mApzc(aApzc)
   {
+    // Make sure the initial velocity is zero. This is normally the case
+    // since we've just stopped a fling, but in some corner cases involving
+    // handoff it could not be.
+    mApzc.mX.SetVelocity(0);
+    mApzc.mY.SetVelocity(0);
   }
 
   virtual bool Sample(FrameMetrics& aFrameMetrics,
@@ -1767,6 +1772,13 @@ bool AsyncPanZoomController::CallDispatchScroll(const ScreenPoint& aStartPoint,
                                           aOverscrollHandoffChainIndex);
 }
 
+void AsyncPanZoomController::CallSnapBackOverscrolledApzc() {
+  APZCTreeManager* treeManagerLocal = mTreeManager;
+  if (treeManagerLocal) {
+    treeManagerLocal->SnapBackOverscrolledApzc(this);
+  }
+}
+
 void AsyncPanZoomController::TrackTouch(const MultiTouchInput& aEvent) {
   ScreenIntPoint prevTouchPoint(mX.GetPos(), mY.GetPos());
   ScreenIntPoint touchPoint = GetFirstTouchScreenPoint(aEvent);
@@ -1957,6 +1969,16 @@ void AsyncPanZoomController::FlushRepaintForOverscrollHandoff() {
   UpdateSharedCompositorFrameMetrics();
 }
 
+bool AsyncPanZoomController::SnapBackIfOverscrolled() {
+  ReentrantMonitorAutoEnter lock(mMonitor);
+  if (IsOverscrolled()) {
+    APZC_LOG("%p is overscrolled, starting snap-back\n", this);
+    StartSnapBack();
+    return true;
+  }
+  return false;
+}
+
 bool AsyncPanZoomController::IsPannable() const {
   ReentrantMonitorAutoEnter lock(mMonitor);
   return mX.CanScroll() || mY.CanScroll();

gfx/layers/apz/src/AsyncPanZoomController.h

@@ -903,6 +903,12 @@ public:
 
   void FlushRepaintForOverscrollHandoff();
 
+  /**
+   * If overscrolled, start a snap-back animation and return true.
+   * Otherwise return false.
+   */
+  bool SnapBackIfOverscrolled();
+
 private:
   FrameMetrics::ViewID mScrollParentId;
 
@@ -914,6 +920,12 @@ private:
   bool CallDispatchScroll(const ScreenPoint& aStartPoint, const ScreenPoint& aEndPoint,
                           uint32_t aOverscrollHandoffChainIndex);
 
+  /**
+   * A similar helper function for calling
+   * APZCTreeManager::SnapBackOverscrolledApzc().
+   */
+  void CallSnapBackOverscrolledApzc();
+
   /**
    * Try to overscroll by 'aOverscroll'.
    * If we are pannable, 'aOverscroll' is added to any existing overscroll,