Bug 1153711 - Do not discard a component of a fling if an APZC further in the handoff chain has room to scroll in that direction. r=kats

gfx/layers/apz/src/AsyncPanZoomController.cpp

@@ -434,17 +434,17 @@ public:
     MOZ_ASSERT(mOverscrollHandoffChain);
     TimeStamp now = AsyncPanZoomController::GetFrameTime();
 
-    // Drop any velocity on axes where we don't have room to scroll anyways.
+    // Drop any velocity on axes where we don't have room to scroll anyways
+    // (in this APZC, or an APZC further in the handoff chain).
     // This ensures that we don't take the 'overscroll' path in Sample()
     // on account of one axis which can't scroll having a velocity.
-    {
+    if (!mOverscrollHandoffChain->CanScrollInDirection(&mApzc, Layer::HORIZONTAL)) {
       ReentrantMonitorAutoEnter lock(mApzc.mMonitor);
-      if (!mApzc.mX.CanScroll()) {
-        mApzc.mX.SetVelocity(0);
-      }
-      if (!mApzc.mY.CanScroll()) {
-        mApzc.mY.SetVelocity(0);
-      }
+      mApzc.mX.SetVelocity(0);
+    }
+    if (!mOverscrollHandoffChain->CanScrollInDirection(&mApzc, Layer::VERTICAL)) {
+      ReentrantMonitorAutoEnter lock(mApzc.mMonitor);
+      mApzc.mY.SetVelocity(0);
     }
 
     ParentLayerPoint velocity = mApzc.GetVelocityVector();
@@ -1484,6 +1484,17 @@ AsyncPanZoomController::CanScrollWithWheel(const LayoutDevicePoint& aDelta) cons
   return false;
 }
 
+bool
+AsyncPanZoomController::CanScroll(Layer::ScrollDirection aDirection) const
+{
+  ReentrantMonitorAutoEnter lock(mMonitor);
+  switch (aDirection) {
+  case Layer::HORIZONTAL: return mX.CanScroll();
+  case Layer::VERTICAL:   return mY.CanScroll();
+  default:                MOZ_ASSERT(false); return false;
+  }
+}
+
 bool
 AsyncPanZoomController::AllowScrollHandoffInWheelTransaction() const
 {

gfx/layers/apz/src/AsyncPanZoomController.h

@@ -22,6 +22,7 @@
 #include "Axis.h"
 #include "InputQueue.h"
 #include "APZUtils.h"
+#include "Layers.h"                     // for Layer::ScrollDirection
 #include "LayersTypes.h"
 #include "TaskThrottler.h"
 #include "mozilla/gfx/Matrix.h"
@@ -370,6 +371,10 @@ public:
   // direction.
   bool CanScrollWithWheel(const LayoutDevicePoint& aDelta) const;
 
+  // Return whether or not there is room to scroll this APZC
+  // in the given direction.
+  bool CanScroll(Layer::ScrollDirection aDirection) const;
+
   void NotifyMozMouseScrollEvent(const nsString& aString) const;
 
 protected:

gfx/layers/apz/src/OverscrollHandoffState.cpp

@@ -143,6 +143,24 @@ OverscrollHandoffChain::CanBePanned(const AsyncPanZoomController* aApzc) const
   return false;
 }
 
+bool
+OverscrollHandoffChain::CanScrollInDirection(const AsyncPanZoomController* aApzc,
+                                             Layer::ScrollDirection aDirection) const
+{
+  // Find |aApzc| in the handoff chain.
+  uint32_t i = IndexOf(aApzc);
+
+  // See whether any APZC in the handoff chain starting from |aApzc|
+  // has room to scroll in the given direction.
+  for (uint32_t j = i; j < Length(); ++j) {
+    if (mChain[j]->CanScroll(aDirection)) {
+      return true;
+    }
+  }
+
+  return false;
+}
+
 bool
 OverscrollHandoffChain::HasOverscrolledApzc() const
 {

gfx/layers/apz/src/OverscrollHandoffState.h

@@ -10,8 +10,9 @@
 #include <vector>
 #include "nsAutoPtr.h"
 #include "nsISupportsImpl.h"  // for NS_INLINE_DECL_REFCOUNTING
-#include "Units.h"            // for ScreenPoint
 #include "APZUtils.h"         // for CancelAnimationFlags
+#include "Layers.h"           // for Layer::ScrollDirection
+#include "Units.h"            // for ScreenPoint
 
 namespace mozilla {
 namespace layers {
@@ -107,6 +108,11 @@ public:
   // has room to be panned.
   bool CanBePanned(const AsyncPanZoomController* aApzc) const;
 
+  // Determine whether the given APZC, or any APZC further in the chain,
+  // can scroll in the given direction.
+  bool CanScrollInDirection(const AsyncPanZoomController* aApzc,
+                            Layer::ScrollDirection aDirection) const;
+
   // Determine whether any APZC along this handoff chain is overscrolled.
   bool HasOverscrolledApzc() const;