Hi,
accoriding to my analysis the event is coming to LayouView,
but in ViewPainter::PaintBoxDecorationBackground() it is returning in between at

  if (DrawingRecorder::UseCachedDrawingIfPossible(
          context, *display_item_client, DisplayItem::kDocumentBackground))
    return;

for RLS, i tried to comment the condition and return, but the background was not consistant

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/41ecce8b3fefc4d967b1ea183ffb3c67bc0f3ac2

commit 41ecce8b3fefc4d967b1ea183ffb3c67bc0f3ac2
Author: Philip Rogers <pdr@chromium.org>
Date: Wed Nov 15 01:44:45 2017

[root layer scrolls] Invalidate on LayoutView style changes

The document element propagates styles to the LayoutView (see
Document::PropagateStyleToViewport) but the LayoutView was early-
outing before fully handling these style changes. In particular,
background color changes did not set
SetBackgroundChangedSinceLastPaintInvalidation.

It does not seem like this code needs to early-out when !parent.
This early-out was added in [1].

[1] https://chromium.googlesource.com/chromium/src/+/a5f6dc8469d54164bc957968cf1a5fde441e4f23

Bug:  781419 ,  777506 

Cq-Include-Trybots: master.tryserver.chromium.linux:linux_layout_tests_root_layer_scrolls;master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2
Change-Id: I6a1e7d28e9dc082a1dac9ba6584ecf7c1efeb9bd
Reviewed-on: https://chromium-review.googlesource.com/768334
Commit-Queue: Philip Rogers <pdr@chromium.org>
Reviewed-by: Steve Kobes <skobes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#516552}
[modify] https://crrev.com/41ecce8b3fefc4d967b1ea183ffb3c67bc0f3ac2/third_party/WebKit/LayoutTests/FlagExpectations/root-layer-scrolls
[modify] https://crrev.com/41ecce8b3fefc4d967b1ea183ffb3c67bc0f3ac2/third_party/WebKit/LayoutTests/flag-specific/root-layer-scrolls/paint/invalidation/background/background-image-paint-invalidation-expected.txt
[modify] https://crrev.com/41ecce8b3fefc4d967b1ea183ffb3c67bc0f3ac2/third_party/WebKit/LayoutTests/flag-specific/root-layer-scrolls/paint/invalidation/svg/relative-sized-document-scrollbars-expected.txt
[add] https://crrev.com/41ecce8b3fefc4d967b1ea183ffb3c67bc0f3ac2/third_party/WebKit/LayoutTests/flag-specific/root-layer-scrolls/platform/linux/paint/invalidation/position/positioned-document-element-expected.txt
[add] https://crrev.com/41ecce8b3fefc4d967b1ea183ffb3c67bc0f3ac2/third_party/WebKit/LayoutTests/flag-specific/root-layer-scrolls/platform/mac/paint/invalidation/position/positioned-document-element-expected.txt
[add] https://crrev.com/41ecce8b3fefc4d967b1ea183ffb3c67bc0f3ac2/third_party/WebKit/LayoutTests/flag-specific/root-layer-scrolls/platform/win/paint/invalidation/position/positioned-document-element-expected.txt
[modify] https://crrev.com/41ecce8b3fefc4d967b1ea183ffb3c67bc0f3ac2/third_party/WebKit/Source/core/layout/LayoutObject.cpp

All tests in fast/selectors are now passing with RLS.