Bug 1749352 - Always call ScrollSnapToDestination from OnPanEnd, and ignore mFollowedByMomentum. r=botond

This patch makes us use the same code path for scroll snapping of
zero-velocity OnPanEnd and followed-by-momentum OnPanEnd.
This is preferable because mFollowedByMomentum is unreliable.

Depends on D135921

Differential Revision: https://phabricator.services.mozilla.com/D135922

gfx/layers/apz/src/AsyncPanZoomController.cpp

@@ -2754,10 +2754,7 @@ nsEventStatus AsyncPanZoomController::OnPanEnd(const PanGestureInput& aEvent) {
   }
 
   RequestContentRepaint();
-
-  if (!aEvent.mFollowedByMomentum) {
-    ScrollSnap();
-  }
+  ScrollSnapToDestination();
 
   return nsEventStatus_eConsumeNoDefault;
 }
@@ -2771,7 +2768,6 @@ nsEventStatus AsyncPanZoomController::OnPanMomentumStart(
   }
 
   SetState(PAN_MOMENTUM);
-  ScrollSnapToDestination();
 
   // Call into OnPan in order to process any delta included in this event.
   OnPan(aEvent, FingersOnTouchpad::No);

gfx/layers/apz/test/gtest/TestSnappingOnMomentum.cpp

@@ -66,8 +66,10 @@ TEST_F(APZCSnappingOnMomentumTesterMock, Snap_On_Momentum) {
   PanGesture(PanGestureInput::PANGESTURE_END, manager, ScreenIntPoint(50, 80),
              ScreenPoint(0, 0), mcc->Time());
 
-  // After lifting the fingers, the velocity should still be positive.
-  EXPECT_GT(apzc->GetVelocityVector().y, 3.0);
+  // After lifting the fingers, the velocity should be zero and a smooth
+  // animation should have been triggered for scroll snap.
+  EXPECT_EQ(apzc->GetVelocityVector().y, 0);
+  apzc->AssertStateIsSmoothMsdScroll();
 
   mcc->AdvanceByMillis(5);