Bug 1749352 - Don't interrupt scroll snapping animations when we receive a momentum pan event. r=botond

Scroll snapping uses an MSD smooth scroll to animate to the target position.
During the scroll snap animation, any synthesized momentum events
from the OS should be ignored; the scroll snap animation has taken over.

It is possible that this patch has unfortunate consequences when mixing
keyboard and touchpad scrolling. For example, pressing space to scroll down
by a page and then doing a fling with the touchpad might now have different
results.

Differential Revision: https://phabricator.services.mozilla.com/D135920

gfx/layers/apz/src/AsyncPanZoomController.cpp

@@ -2766,12 +2766,7 @@ nsEventStatus AsyncPanZoomController::OnPanMomentumStart(
     const PanGestureInput& aEvent) {
   APZC_LOG("%p got a pan-momentumstart in state %d\n", this, mState);
 
-  if (mState == SMOOTHMSD_SCROLL) {
-    // SMOOTHMSD_SCROLL scrolls are cancelled by pan gestures.
-    CancelAnimation();
-  }
-
-  if (mState == OVERSCROLL_ANIMATION) {
+  if (mState == SMOOTHMSD_SCROLL || mState == OVERSCROLL_ANIMATION) {
     return nsEventStatus_eConsumeNoDefault;
   }