Bug 1676791 - Part 7: Disable OMTA for scroll-timeline. r=hiro

For simplicity purposes, we don't consider OMTA for now. OMTA support
for scoll-timeline will be done in Bug 1737180.

However, in this patch, we would like to address:
if the geometic animations use scroll-timeline, we don't have to let it
affect the transform animations on the same animation target:
1. If we don't do scrolling, its geometric properties don't change, so no
   effect on transform-like properties.
2. If we do scrolling, we may un-throttle the transform animations by
   other ways, e.g. Keyframe::CanThrottle(). So we don't need to worry
   about this.

Note: tests are in the next patch.

Differential Revision: https://phabricator.services.mozilla.com/D129103

dom/animation/KeyframeEffect.cpp

@@ -1687,9 +1687,13 @@ bool KeyframeEffect::ShouldBlockAsyncTransformAnimations(
     return true;
   }
 
+  MOZ_ASSERT(mAnimation);
+  // Note: If the geometric animations are using scroll-timeline, we don't need
+  // to synchronize transform animations with them.
   const bool enableMainthreadSynchronizationWithGeometricAnimations =
       StaticPrefs::
-          dom_animations_mainthread_synchronization_with_geometric_animations();
+          dom_animations_mainthread_synchronization_with_geometric_animations() &&
+      !mAnimation->UsingScrollTimeline();
 
   for (const AnimationProperty& property : mProperties) {
     // If there is a property for animations level that is overridden by
@@ -2025,6 +2029,13 @@ KeyframeEffect::MatchForCompositor KeyframeEffect::IsMatchForCompositor(
     return KeyframeEffect::MatchForCompositor::NoAndBlockThisProperty;
   }
 
+  // Unconditionally disable OMTA for scroll-timeline.
+  // FIXME: Bug 1737180: Once we support OMTA for scroll-timeline, we can just
+  // drop this.
+  if (mAnimation->UsingScrollTimeline()) {
+    return KeyframeEffect::MatchForCompositor::No;
+  }
+
   if (!HasEffectiveAnimationOfPropertySet(aPropertySet, aEffects)) {
     return KeyframeEffect::MatchForCompositor::No;
   }