Narrow Bisect:
https://chromium.googlesource.com/chromium/src/+log/b1efd1e7b6d9eb46c2a367a84f8350623093ec14..8e641dfb7ee85c52e2e7957ab26c102cc9dd67f9?pretty=fuller&n=10000

Suspect: r480803?

@hayleyferr:  Could you please check whether this is caused with respect to your change, if not please help us in assigning it to the right owner.

@alph: mind taking a look whether the platform is doing something wrong or we use it incorrectly?

over to reviewer...

pdr@, could you take a look since you've been looking at scrolling recently. I can verify that the window doesn't scroll when the mouse is over the scrollbar. Note that if the mouse is over the content of the window then it has a custom action of zooming in instead of scrolling.

This is a bug with custom scrollbars and mouse-wheel scrolling. Here's a minimized testcase: http://output.jsbin.com/viheyek/quiet

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/176756e93d0cf910e461547f6c7c0e7bfbce36a3

commit 176756e93d0cf910e461547f6c7c0e7bfbce36a3
Author: Philip Rogers <pdr@chromium.org>
Date: Thu Oct 12 22:59:39 2017

Ensure LayerImpls that draw content are hit testable

[1] introduced a bit on LayerImpl for hit testing layers that do not
draw content. There was a bug in how this bit was pushed (Layer to
LayerImpl) where a Layer that draws content would have a LayerImpl
that was not marked as being hit testable because the (false)
should_hit_test bit was pushed after the (true) draws_content bit.

This patch renames should_hit_test to hit_testable_without_draws_content
and makes should_hit_test depend on both drawing content and
hit_testable_without_draws_content. A test has been added that covers
the Layer->LayerImpl property push behavior.

[1] https://chromium-review.googlesource.com/c/chromium/src/+/529426

Bug:  764271 
Cq-Include-Trybots: master.tryserver.blink:linux_trusty_blink_rel;master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2
Change-Id: Id5d9f2cf9cd6a5a6e33808b3729bf3c16f481037
Reviewed-on: https://chromium-review.googlesource.com/711136
Reviewed-by: vmpstr <vmpstr@chromium.org>
Reviewed-by: Xida Chen <xidachen@chromium.org>
Commit-Queue: Philip Rogers <pdr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#508519}
[modify] https://crrev.com/176756e93d0cf910e461547f6c7c0e7bfbce36a3/cc/blink/web_layer_impl.cc
[modify] https://crrev.com/176756e93d0cf910e461547f6c7c0e7bfbce36a3/cc/blink/web_layer_impl.h
[modify] https://crrev.com/176756e93d0cf910e461547f6c7c0e7bfbce36a3/cc/layers/layer.cc
[modify] https://crrev.com/176756e93d0cf910e461547f6c7c0e7bfbce36a3/cc/layers/layer.h
[modify] https://crrev.com/176756e93d0cf910e461547f6c7c0e7bfbce36a3/cc/layers/layer_impl.cc
[modify] https://crrev.com/176756e93d0cf910e461547f6c7c0e7bfbce36a3/cc/layers/layer_impl.h
[modify] https://crrev.com/176756e93d0cf910e461547f6c7c0e7bfbce36a3/cc/layers/layer_unittest.cc
[modify] https://crrev.com/176756e93d0cf910e461547f6c7c0e7bfbce36a3/cc/trees/layer_tree_host_impl_unittest.cc
[modify] https://crrev.com/176756e93d0cf910e461547f6c7c0e7bfbce36a3/third_party/WebKit/Source/core/paint/compositing/CompositedLayerMapping.cpp
[modify] https://crrev.com/176756e93d0cf910e461547f6c7c0e7bfbce36a3/third_party/WebKit/Source/core/paint/compositing/CompositedLayerMapping.h
[modify] https://crrev.com/176756e93d0cf910e461547f6c7c0e7bfbce36a3/third_party/WebKit/Source/core/paint/compositing/CompositedLayerMappingTest.cpp
[modify] https://crrev.com/176756e93d0cf910e461547f6c7c0e7bfbce36a3/third_party/WebKit/Source/platform/graphics/GraphicsLayer.cpp
[modify] https://crrev.com/176756e93d0cf910e461547f6c7c0e7bfbce36a3/third_party/WebKit/Source/platform/graphics/GraphicsLayer.h
[modify] https://crrev.com/176756e93d0cf910e461547f6c7c0e7bfbce36a3/third_party/WebKit/public/platform/WebLayer.h