After some analysis, it looks like we're okay for all the height changes that happen above the layout viewport, but will jump for cases where the height changes happen below the anchor element but above the visual viewport.

I think the right thing to do is to ensure that the selected anchor is within the visual viewport rather than the layout viewport.

This sounds right to me.  There is also the question of which viewport to scroll for the adjustment.

Arguably if we are selecting an anchor node based on the visual viewport, we should distribute the adjustment through RootFrameViewport.  In that case RootFrameViewport should own the ScrollAnchor instead of FrameView.

I think tdresser mentioned in the initial design discussion that it may be preferable for the adjustment to scroll the layout viewport first (exclusively?) so that fixed-position elements don't jump.  That might require new plumbing in RootFrameViewport.

Re #7, I think moving the layout viewport first is correct, for the reason described.

Agree that we should be anchoring using the visual viewport and scrolling the layout viewport first.

Does scroll anchoring deal with cases where elements are removed from the page or is it just for adding elements? If it's just the latter than I think scrolling just the layout viewport should be enough since you can't get into a situation where you *must* scroll the visual viewport. The former does have those cases and would likely some minor changes in RootFrameViewport.

Scroll anchoring potentially reacts to any layout change (adding, removing, resizing, changing styles, etc).

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/577f3d408ca3e63ff1a9efc0a95f098df5092fd6

commit 577f3d408ca3e63ff1a9efc0a95f098df5092fd6
Author: ymalik <ymalik@chromium.org>
Date: Fri Jul 08 22:18:02 2016

Improve scroll anchoring's interactions with the visual viewport

ScrollAnchor can now have a RootFrameViewport as its scroller so
that an anchoring adjustment can be distributed between layout
and visual viewports.

BUG= 594876 

Review-Url: https://codereview.chromium.org/2123053002
Cr-Commit-Position: refs/heads/master@{#404508}

[modify] https://crrev.com/577f3d408ca3e63ff1a9efc0a95f098df5092fd6/third_party/WebKit/Source/core/frame/FrameView.cpp
[modify] https://crrev.com/577f3d408ca3e63ff1a9efc0a95f098df5092fd6/third_party/WebKit/Source/core/frame/FrameView.h
[modify] https://crrev.com/577f3d408ca3e63ff1a9efc0a95f098df5092fd6/third_party/WebKit/Source/core/frame/RootFrameViewport.cpp
[modify] https://crrev.com/577f3d408ca3e63ff1a9efc0a95f098df5092fd6/third_party/WebKit/Source/core/frame/RootFrameViewport.h
[modify] https://crrev.com/577f3d408ca3e63ff1a9efc0a95f098df5092fd6/third_party/WebKit/Source/core/frame/VisualViewport.cpp
[modify] https://crrev.com/577f3d408ca3e63ff1a9efc0a95f098df5092fd6/third_party/WebKit/Source/core/frame/VisualViewport.h
[modify] https://crrev.com/577f3d408ca3e63ff1a9efc0a95f098df5092fd6/third_party/WebKit/Source/core/layout/ScrollAnchor.cpp
[modify] https://crrev.com/577f3d408ca3e63ff1a9efc0a95f098df5092fd6/third_party/WebKit/Source/core/layout/ScrollAnchor.h
[modify] https://crrev.com/577f3d408ca3e63ff1a9efc0a95f098df5092fd6/third_party/WebKit/Source/core/layout/ScrollAnchorTest.cpp
[modify] https://crrev.com/577f3d408ca3e63ff1a9efc0a95f098df5092fd6/third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.h
[modify] https://crrev.com/577f3d408ca3e63ff1a9efc0a95f098df5092fd6/third_party/WebKit/Source/platform/scroll/ProgrammaticScrollAnimator.cpp
[modify] https://crrev.com/577f3d408ca3e63ff1a9efc0a95f098df5092fd6/third_party/WebKit/Source/platform/scroll/ScrollAnimator.cpp
[modify] https://crrev.com/577f3d408ca3e63ff1a9efc0a95f098df5092fd6/third_party/WebKit/Source/platform/scroll/ScrollAnimator.h
[modify] https://crrev.com/577f3d408ca3e63ff1a9efc0a95f098df5092fd6/third_party/WebKit/Source/platform/scroll/ScrollAnimatorBase.cpp
[modify] https://crrev.com/577f3d408ca3e63ff1a9efc0a95f098df5092fd6/third_party/WebKit/Source/platform/scroll/ScrollAnimatorCompositorCoordinator.cpp
[modify] https://crrev.com/577f3d408ca3e63ff1a9efc0a95f098df5092fd6/third_party/WebKit/Source/platform/scroll/ScrollAnimatorCompositorCoordinator.h
[modify] https://crrev.com/577f3d408ca3e63ff1a9efc0a95f098df5092fd6/third_party/WebKit/Source/platform/scroll/ScrollAnimatorTest.cpp
[modify] https://crrev.com/577f3d408ca3e63ff1a9efc0a95f098df5092fd6/third_party/WebKit/Source/platform/scroll/ScrollableArea.cpp
[modify] https://crrev.com/577f3d408ca3e63ff1a9efc0a95f098df5092fd6/third_party/WebKit/Source/platform/scroll/ScrollableArea.h

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/ed3cc7f461c0a5c6ad66e509deace2e07674537c

commit ed3cc7f461c0a5c6ad66e509deace2e07674537c
Author: ymalik <ymalik@chromium.org>
Date: Tue Jul 19 15:19:25 2016

Make scroll anchoring adjustment when the bounds of a scroller is changed

A layout update can change the bounds of a scroller, requiring us to clamp the
current scroll position if its outside the bounds of the scroller. This would
clear the saved scroll anchor and have a visible jump. The main issue is that
we would clear the anchor between ScrollAnchor::save and ScrollAnchor::restore.

This CL calls ::restore if clamping is needed to ensure that scroll anchoring
still works.

BUG= 594876 

Review-Url: https://codereview.chromium.org/2152773002
Cr-Commit-Position: refs/heads/master@{#406277}

[modify] https://crrev.com/ed3cc7f461c0a5c6ad66e509deace2e07674537c/third_party/WebKit/Source/core/frame/FrameView.cpp
[modify] https://crrev.com/ed3cc7f461c0a5c6ad66e509deace2e07674537c/third_party/WebKit/Source/core/layout/ScrollAnchorTest.cpp
[modify] https://crrev.com/ed3cc7f461c0a5c6ad66e509deace2e07674537c/third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp