Correctly reset ScrollAnimator animation state after cancelling animation

third_party/WebKit/Source/platform/scroll/ScrollAnimator.cpp

 /*
  * Copyright (c) 2011, Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
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@@ skipping 94 matching lines @@
 
     TRACE_EVENT0("blink", "ScrollAnimator::scroll");
 
     if (granularity == ScrollByPrecisePixel) {
         // Cancel scroll animation because asked to instant scroll.
         if (hasRunningAnimation())
             cancelAnimation();
         return ScrollAnimatorBase::userScroll(granularity, delta);
     }
 
+    if (m_runState == RunState::PostAnimationCleanup)
+        resetAnimationState();

[ajuma, 2016/03/29 18:41:57]

What happens to the actual cleanup (specifically, the removal of the main thread
scrolling reason)?

[ymalik, 2016/03/29 19:42:17]

That's a good point (though that's a separate issue from this).

This is the special case when we get a user scroll while we are in the
PostAnimationCleanup state after an animation is cancelled. Note that to remove
the main thread scrolling reason, we need to be in compositing clean.

After the call to willAnimateToOffset below, we will either start a new
animation (or update the current one) or not do anything. If we start a new
animation, we will add main thread scrolling reason anyway, so it's probably
okay to not clear it here. We were not handling the latter, which I think is
fixed if we stay in the PostAnimationCleanup state. Does that make sense?

+
     FloatSize consumedDelta = computeDeltaToConsume(delta);
 
     FloatPoint targetPos = desiredTargetPosition();
     targetPos.move(consumedDelta);
 
     if (willAnimateToOffset(targetPos)) {
         m_lastGranularity = granularity;
         // Report unused delta only if there is no animation running. See
         // comment below regarding scroll latching.
         // TODO(bokan): Need to standardize how ScrollAnimators report
@@ skipping 277 matching lines @@
     }
     return true;
 }
 
 DEFINE_TRACE(ScrollAnimator)
 {
     ScrollAnimatorBase::trace(visitor);
 }
 
 } // namespace blink