Tie scroll anchoring adjustments to frame lifecycle instead of layout.

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

 /*
  * Copyright (c) 2010, Google Inc. All rights reserved.
  * Copyright (C) 2008, 2011 Apple 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:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
@@ skipping 166 matching lines @@
                                      ScrollBehavior behavior) {
   ScrollOffset clampedOffset = clampScrollOffset(offset);
   if (clampedOffset == scrollOffset())
     return;
 
   if (behavior == ScrollBehaviorAuto)
     behavior = scrollBehaviorStyle();
 
   switch (scrollType) {
     case CompositorScroll:
+    case ClampingScroll:

[szager1, 2016/10/13 17:54:49]

I would have thought this would be treated like a ProgrammaticScroll, no?

[skobes, 2016/10/13 18:00:31]

Generally it is.  The important difference is that we don't clear the scroll
anchor.

We also can't use programmaticScrollHelper here because
ProgrammaticScrollAnimator will pass ProgrammaticScroll to scrollOffsetChanged.

[szager1, 2016/10/13 18:50:56]

This code path doesn't call cancelScrollAnimation(), is that intended?

[bokan, 2016/10/13 18:53:59]

Perhaps programmaticScrollHelper should take a scrollType? When I added it there
was just user and programmatic scrolls.

[skobes, 2016/10/13 18:55:19]

Basically, yes.  I assumed that clamping cancelling animation was an unintended
side effect of reusing the ProgrammaticScroll value.  I can't think of a
scenario where it is important for clamping to cancel animation.

[szager1, 2016/10/13 19:00:39]

I haven't thought too deeply about this, but my instinct is that whenever a
discontinuous scroll happens (including programmatic and clamping scrolls), we
should cancel animation.  Scroll anchoring could be a legitimate exception to
that, though.

       scrollOffsetChanged(clampedOffset, scrollType);
       break;
     case AnchoringScroll:
       scrollAnimator().adjustAnimationAndSetScrollOffset(clampedOffset,
                                                          scrollType);
       break;
     case ProgrammaticScroll:
       programmaticScrollHelper(clampedOffset, behavior);
       break;
     case UserScroll:
@@ skipping 393 matching lines @@
   return IntSize(std::max(0, size.width() - verticalScrollbarWidth()),
                  std::max(0, size.height() - horizontalScrollbarHeight()));
 }
 
 DEFINE_TRACE(ScrollableArea) {
   visitor->trace(m_scrollAnimator);
   visitor->trace(m_programmaticScrollAnimator);
 }
 
 }  // namespace blink