This is because LayerTreeHostImpl::ScrollAnimated doesn't really have a notion of distributing scroll between the inner and outer viewports. It simply applies all of the delta without scaling to the InnerViewport without scaling. 

We probably need to do something like this https://code.google.com/p/chromium/codesearch#chromium/src/cc/layers/viewport.cc&rcl=1458143321&l=35

This is probably not super urgent since the number of people that pinch-zoom in and use the mousewheel to scroll is not that high.

 Issue 575019  is closely related.

Can we not defer to Main Thread scrolling if the viewport has a scale? So at least the user behavior is correct?

I don't understand how this is related to viewport distribution at all. The delta should be scaled before it even makes it to the scroll animator or viewport distribution.

At #4, that's probably a simple solution for now. We probably want to do what  Issue 575019  suggests though.

At #5, This is for the impl thread. The way I understand it, the scaling actually happens after a call to LTHI::ScrollBy (see https://code.google.com/p/chromium/codesearch#chromium/src/cc/trees/layer_tree_host_impl.cc&rcl=1458199924&l=2932)

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/99740e85f5ebeb08b80a13b34144f7f7f2d1687e

commit 99740e85f5ebeb08b80a13b34144f7f7f2d1687e
Author: ymalik <ymalik@chromium.org>
Date: Thu Apr 07 04:18:13 2016

Connect LTHI::ScrollAnimated to cc::Viewport

This CL removed to hack in FindScrollLayerForDeviceViewportPoint to ensure
that we scroll the outer viewport and connects the LTHI::ScrollAnimated function
to the viewport class.

The viewport class distributes the scroll delta between the inner and outer
viewports and starts animation the layer which consumes more of the scroll delta.
Our animation system does not support running two scroll offset animations and this
is something that we may need to add in future.

BUG= 575019 , 595421 
CQ_INCLUDE_TRYBOTS=tryserver.blink:linux_blink_rel

Review URL: https://codereview.chromium.org/1866583002

Cr-Commit-Position: refs/heads/master@{#385657}

[modify] https://crrev.com/99740e85f5ebeb08b80a13b34144f7f7f2d1687e/cc/BUILD.gn
[modify] https://crrev.com/99740e85f5ebeb08b80a13b34144f7f7f2d1687e/cc/cc_tests.gyp
[modify] https://crrev.com/99740e85f5ebeb08b80a13b34144f7f7f2d1687e/cc/input/input_handler.h
[modify] https://crrev.com/99740e85f5ebeb08b80a13b34144f7f7f2d1687e/cc/layers/viewport.cc
[modify] https://crrev.com/99740e85f5ebeb08b80a13b34144f7f7f2d1687e/cc/layers/viewport.h
[add] https://crrev.com/99740e85f5ebeb08b80a13b34144f7f7f2d1687e/cc/layers/viewport_unittest.cc
[modify] https://crrev.com/99740e85f5ebeb08b80a13b34144f7f7f2d1687e/cc/trees/layer_tree_host_impl.cc
[modify] https://crrev.com/99740e85f5ebeb08b80a13b34144f7f7f2d1687e/cc/trees/layer_tree_host_impl.h
[modify] https://crrev.com/99740e85f5ebeb08b80a13b34144f7f7f2d1687e/cc/trees/layer_tree_host_impl_unittest.cc
[modify] https://crrev.com/99740e85f5ebeb08b80a13b34144f7f7f2d1687e/cc/trees/property_tree.h