Bug 1833950 - Handle duplicate pan-end on fling state too. r=botond

Differential Revision: https://phabricator.services.mozilla.com/D178483

gfx/layers/apz/src/AsyncPanZoomController.cpp

@@ -1853,7 +1853,7 @@ nsEventStatus AsyncPanZoomController::OnScaleEnd(
 }
 
 nsEventStatus AsyncPanZoomController::HandleEndOfPan() {
-  MOZ_ASSERT(mAnimation == nullptr);
+  MOZ_ASSERT(!mAnimation);
   MOZ_ASSERT(GetCurrentTouchBlock() || GetCurrentPanGestureBlock());
   GetCurrentInputBlock()->GetOverscrollHandoffChain()->FlushRepaints();
   ParentLayerPoint flingVelocity = GetVelocityVector();
@@ -2883,11 +2883,12 @@ nsEventStatus AsyncPanZoomController::OnPanEnd(const PanGestureInput& aEvent) {
   APZC_LOG_DETAIL("got a pan-end in state %s\n", this,
                   ToString(mState).c_str());
 
-  // This can happen if the OS sends a second pan-end event after
-  // the first one has already started an overscroll animation.
+  // This can happen if the OS sends a second pan-end event after the first one
+  // has already started an overscroll animation or entered a fling state.
   // This has been observed on some Wayland versions.
   PanZoomState currentState = GetState();
-  if (currentState == OVERSCROLL_ANIMATION || currentState == NOTHING) {
+  if (currentState == OVERSCROLL_ANIMATION || currentState == NOTHING ||
+      currentState == FLING) {
     return nsEventStatus_eIgnore;
   }