Assuming that the intervention might be problematic (TBC), for V1, instead of trying to make anchoring work flawlessly while scrolling, I suggest that we consider keeping the statu quo (i.e. have the anchoring temporarily disabled until the gesture ends?) if it's simpler and/or less risky.

The status quo is that scroll anchoring has no idea whether we are in the middle of a scroll gesture.  I haven't investigated exactly what the consequence of this is.  It's possible the gesture will just override the anchor.

The problem of anchoring mid-gesture is actually easier than the general scroll anchoring problem: since we have already hit-tested the point under the user's finger, we have a natural candidate for the anchor object.

So I think we are in decent shape for touch gestures because the GestureScrollUpdate events generate deltas, so they compose correctly with anchoring scrolls.

Arguably we should use the event target as the anchor node, but that is a low priority optimization.

The hard case is actually smooth scrolling, because the animation curve is giving us absolute scroll offsets.  I guess the right thing is for the anchoring scroll to adjust both initial and target values of the in-progress animation, with attendant plumbing to the compositor thread.

Without this, scroll anchoring is effectively disabled during the smooth scroll. :-/

The main thread part here is not that bad since we can just abort the animation, update the curve, and start another animation. Main thread part = animation that get initiated by MT and run on CC (or MT).

The CC part is a little tricky because we probably need to have some plumbing that tells CC to do the same, or takeover the animation. Resolving  issue 607047 
may give that to us for free.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/98eaa26e00d063ccf9f31cd85261f66441bcd5b8

commit 98eaa26e00d063ccf9f31cd85261f66441bcd5b8
Author: ymalik <ymalik@chromium.org>
Date: Mon May 02 18:21:22 2016

Smooth scroll animation should not override scroll anchoring update (MT)

Previously, the adjustment made by scroll anchoriing would be overwritten by
the scroll position updates from the scroll offset animation because it worked
with absolute scroll offset values.

Now, when an adjustment is about to be made with an ongoing animation initiated
by MT, we cancel that animation, update the curve, and schedule an animatiion
with the update target. We do this by introducing a new state to the
ScrollAnimator class which also fixes  issue 599876 .

BUG= 594456 , 599876 

Review-Url: https://codereview.chromium.org/1926473003
Cr-Commit-Position: refs/heads/master@{#390997}

[modify] https://crrev.com/98eaa26e00d063ccf9f31cd85261f66441bcd5b8/third_party/WebKit/LayoutTests/TestExpectations
[add] https://crrev.com/98eaa26e00d063ccf9f31cd85261f66441bcd5b8/third_party/WebKit/LayoutTests/fast/scroll-behavior/smooth-scroll/ongoing-smooth-scroll-anchors.html
[modify] https://crrev.com/98eaa26e00d063ccf9f31cd85261f66441bcd5b8/third_party/WebKit/Source/core/layout/ScrollAnchor.cpp
[modify] https://crrev.com/98eaa26e00d063ccf9f31cd85261f66441bcd5b8/third_party/WebKit/Source/platform/scroll/ScrollAnimator.cpp
[modify] https://crrev.com/98eaa26e00d063ccf9f31cd85261f66441bcd5b8/third_party/WebKit/Source/platform/scroll/ScrollAnimatorCompositorCoordinator.cpp
[modify] https://crrev.com/98eaa26e00d063ccf9f31cd85261f66441bcd5b8/third_party/WebKit/Source/platform/scroll/ScrollAnimatorCompositorCoordinator.h
[modify] https://crrev.com/98eaa26e00d063ccf9f31cd85261f66441bcd5b8/third_party/WebKit/Source/platform/scroll/ScrollAnimatorTest.cpp

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/1b8eeda89c1ddb96201d98c9446afd5da2d68c18

commit 1b8eeda89c1ddb96201d98c9446afd5da2d68c18
Author: ymalik <ymalik@chromium.org>
Date: Wed May 04 16:18:17 2016

Rename and refactor ScrollOffsetAnimations to ScrollOffsetAnimationsImpl

Pre-req CL to enable communication between MT and impl-only animations. This
is described in http://bit.ly/1Y6RA6y.

Note that this CL has no behavioral change.

BUG= 594456 
CQ_INCLUDE_TRYBOTS=tryserver.blink:linux_blink_rel

Review-Url: https://codereview.chromium.org/1952563002
Cr-Commit-Position: refs/heads/master@{#391530}

[modify] https://crrev.com/1b8eeda89c1ddb96201d98c9446afd5da2d68c18/cc/BUILD.gn
[modify] https://crrev.com/1b8eeda89c1ddb96201d98c9446afd5da2d68c18/cc/animation/animation_host.cc
[modify] https://crrev.com/1b8eeda89c1ddb96201d98c9446afd5da2d68c18/cc/animation/animation_host.h
[add] https://crrev.com/1b8eeda89c1ddb96201d98c9446afd5da2d68c18/cc/animation/scroll_offset_animations_impl.cc
[add] https://crrev.com/1b8eeda89c1ddb96201d98c9446afd5da2d68c18/cc/animation/scroll_offset_animations_impl.h
[modify] https://crrev.com/1b8eeda89c1ddb96201d98c9446afd5da2d68c18/cc/cc.gyp

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/f3a9e63578aff4d5c814a5bb374665a4a0812834

commit f3a9e63578aff4d5c814a5bb374665a4a0812834
Author: ymalik <ymalik@chromium.org>
Date: Fri May 20 01:31:07 2016

Communicate MT changes to impl-only scroll offset animations

This CL adds cc::ScrollOffsetAnimations which is the MT version of
cc::ScrollOffsetAnimationsImpl.

The flow is as follows: The scroll offset is changed on the MT (e.g. scroll
anchoring adjustment) while there is an ongoing impl-only scroll offset
animation. ScrollAnimator::updateImplOnlyAnimationWithAdjustment is called,
which pushes the update to MT's cc::AnimationHost via
CompositorAnimationTimeline::CompositorAnimationHost::AnimationHost.

During commit, cc::ScrollOffsetAnimations::PushPropertiesTo calls the
relevant handler in response to the update. Once the updates are applied,
the list of updates is cleared.

This is described in http://bit.ly/1Y6RA6y.

Note that in this CL, this path is only exercised from ScrollAnchor.cpp

BUG= 594456 
CQ_INCLUDE_TRYBOTS=tryserver.blink:linux_blink_rel

Review-Url: https://codereview.chromium.org/1950243005
Cr-Commit-Position: refs/heads/master@{#394947}

[modify] https://crrev.com/f3a9e63578aff4d5c814a5bb374665a4a0812834/cc/BUILD.gn
[modify] https://crrev.com/f3a9e63578aff4d5c814a5bb374665a4a0812834/cc/animation/animation_host.cc
[modify] https://crrev.com/f3a9e63578aff4d5c814a5bb374665a4a0812834/cc/animation/animation_host.h
[modify] https://crrev.com/f3a9e63578aff4d5c814a5bb374665a4a0812834/cc/animation/scroll_offset_animation_curve.cc
[modify] https://crrev.com/f3a9e63578aff4d5c814a5bb374665a4a0812834/cc/animation/scroll_offset_animation_curve.h
[add] https://crrev.com/f3a9e63578aff4d5c814a5bb374665a4a0812834/cc/animation/scroll_offset_animations.cc
[add] https://crrev.com/f3a9e63578aff4d5c814a5bb374665a4a0812834/cc/animation/scroll_offset_animations.h
[modify] https://crrev.com/f3a9e63578aff4d5c814a5bb374665a4a0812834/cc/animation/scroll_offset_animations_impl.cc
[modify] https://crrev.com/f3a9e63578aff4d5c814a5bb374665a4a0812834/cc/animation/scroll_offset_animations_impl.h
[modify] https://crrev.com/f3a9e63578aff4d5c814a5bb374665a4a0812834/cc/cc.gyp
[modify] https://crrev.com/f3a9e63578aff4d5c814a5bb374665a4a0812834/cc/trees/layer_tree_host_unittest_animation.cc
[modify] https://crrev.com/f3a9e63578aff4d5c814a5bb374665a4a0812834/third_party/WebKit/Source/core/layout/ScrollAnchor.cpp
[add] https://crrev.com/f3a9e63578aff4d5c814a5bb374665a4a0812834/third_party/WebKit/Source/platform/animation/CompositorAnimationHost.cpp
[add] https://crrev.com/f3a9e63578aff4d5c814a5bb374665a4a0812834/third_party/WebKit/Source/platform/animation/CompositorAnimationHost.h
[add] https://crrev.com/f3a9e63578aff4d5c814a5bb374665a4a0812834/third_party/WebKit/Source/platform/animation/CompositorAnimationHostTest.cpp
[modify] https://crrev.com/f3a9e63578aff4d5c814a5bb374665a4a0812834/third_party/WebKit/Source/platform/animation/CompositorAnimationTimeline.cpp
[modify] https://crrev.com/f3a9e63578aff4d5c814a5bb374665a4a0812834/third_party/WebKit/Source/platform/animation/CompositorAnimationTimeline.h
[modify] https://crrev.com/f3a9e63578aff4d5c814a5bb374665a4a0812834/third_party/WebKit/Source/platform/blink_platform.gypi
[modify] https://crrev.com/f3a9e63578aff4d5c814a5bb374665a4a0812834/third_party/WebKit/Source/platform/scroll/ScrollAnimator.cpp
[modify] https://crrev.com/f3a9e63578aff4d5c814a5bb374665a4a0812834/third_party/WebKit/Source/platform/scroll/ScrollAnimatorCompositorCoordinator.cpp
[modify] https://crrev.com/f3a9e63578aff4d5c814a5bb374665a4a0812834/third_party/WebKit/Source/platform/scroll/ScrollAnimatorCompositorCoordinator.h
[modify] https://crrev.com/f3a9e63578aff4d5c814a5bb374665a4a0812834/third_party/WebKit/Source/platform/scroll/ScrollAnimatorTest.cpp

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/07ed51b3771572c4bff43a990351f4be1de4f4ae

commit 07ed51b3771572c4bff43a990351f4be1de4f4ae
Author: ymalik <ymalik@chromium.org>
Date: Tue May 31 20:24:57 2016

Correctly update scroll offset animations in response to scroll anchoring

This CL fixes the incorrect implementation in https://crrev.com/1926473003.
In addition to the state WaitingToCancelOnCompositorButNewScroll introduced
previously, it adds a new state RunningOnCompositorButNeedsAdjustment when
an ongoing animation needs to be adjusted. We need the former to fix
 issue 599876 .

When an adjustment needs to be made, ScrollAnchor calls into ScrollAnimator
which adjusts the curve. When the animation is running on MT, this is enough,
but if the animation is running on CC, we abort the old animation and
push a new one with the adjusted initial and target positions (hence the new
state).

This CL also cleans up ::updateCompositorAnimations.

Added a new test case for testing MT animation and an animation running on CC
is tested by the layout test added in the initial patch.
(fast/scroll-behavior/smooth-scroll/ongoing-smooth-scroll-anchors.html)

BUG= 594456 

Review-Url: https://codereview.chromium.org/2015113003
Cr-Commit-Position: refs/heads/master@{#396913}

[add] https://crrev.com/07ed51b3771572c4bff43a990351f4be1de4f4ae/third_party/WebKit/LayoutTests/fast/scroll-behavior/smooth-scroll/ongoing-smooth-scroll-vertical-rl-anchors.html
[modify] https://crrev.com/07ed51b3771572c4bff43a990351f4be1de4f4ae/third_party/WebKit/Source/core/layout/ScrollAnchor.cpp
[modify] https://crrev.com/07ed51b3771572c4bff43a990351f4be1de4f4ae/third_party/WebKit/Source/platform/animation/CompositorScrollOffsetAnimationCurve.cpp
[modify] https://crrev.com/07ed51b3771572c4bff43a990351f4be1de4f4ae/third_party/WebKit/Source/platform/animation/CompositorScrollOffsetAnimationCurve.h
[modify] https://crrev.com/07ed51b3771572c4bff43a990351f4be1de4f4ae/third_party/WebKit/Source/platform/scroll/ScrollAnimator.cpp
[modify] https://crrev.com/07ed51b3771572c4bff43a990351f4be1de4f4ae/third_party/WebKit/Source/platform/scroll/ScrollAnimator.h
[modify] https://crrev.com/07ed51b3771572c4bff43a990351f4be1de4f4ae/third_party/WebKit/Source/platform/scroll/ScrollAnimatorCompositorCoordinator.cpp
[modify] https://crrev.com/07ed51b3771572c4bff43a990351f4be1de4f4ae/third_party/WebKit/Source/platform/scroll/ScrollAnimatorCompositorCoordinator.h
[modify] https://crrev.com/07ed51b3771572c4bff43a990351f4be1de4f4ae/third_party/WebKit/Source/platform/scroll/ScrollAnimatorTest.cpp