Adding RB Label as this is a recent Regression. Please remove if not required.

Please find the below bisect information,

Good build:54.0.2824.0 (Revision -- 410520).
Bad build:54.0.2825.0 (Revision -- 410913).

You are probably looking for a change made after 410796 (known good), but no later than 410797 (first known bad).
CHANGELOG URL:
  https://chromium.googlesource.com/chromium/src/+log/c330e85e9138e8c8ce6446494802190ca03fcb52..9a2cdc8d96d94c736eb5547a7c8c541e58e5759f

Suspected Commit# 9a2cdc8d96d94c736eb5547a7c8c541e58e5759f
Suspected Review URL# https://codereview.chromium.org/2196583002

@flackr -- Could you please look into the issue. Pardon me if it has nothing to do with your changes and if possible please assign it to concern Dev.
Thank You.

Yes, my change caused this behavior. Investigating.

+chrishtr, the issue here seems to have to do with the combination of promotion due to overlap with other composited elements and a padding-box background (i.e. if I will-change: transform the element it paints correctly). It seems like the safe thing would be to turn off painting into the composited scrolling contents layer in M54. Can we land a targeted patch to M54 to disable this? Or can we revert my patch directly in the 54 branch? It will definitely not cleanly revert on tip of tree anymore.

A targeted removal in M54 sounds good. How about:

1. Land a patch to put paint-background-onto-scrolling behind the promote-low-dpi-scrolling flag
2. Merge #1 to M54

Sounds good.

I've discovered the problem. The issue seems to be that we aren't actually compositing the scrolling contents, amusingly if you run with --enable-blink-features=CompositeOpaqueScrollers the problem goes away because we then enable compositing the scrolling contents because we know we can paint the background into them.

Actually I found the root problem and have a proper fix: https://codereview.chromium.org/2346883003/. Basically, we would set CompositedLayerMapping::shouldPaintBackgroundIntoScrollingContentsLayer() regardless of whether we had a scrolling contents layer, then we would never paint the background because there was no scrolling contents layer painted into.

M54 Stable release is scheduled for the first week of OCT, please have the fix baked/verified in canary and request a merge to M54 ASAP.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/dd59f478410b76764e227fe24642bae6f7b06b26

commit dd59f478410b76764e227fe24642bae6f7b06b26
Author: flackr <flackr@chromium.org>
Date: Fri Sep 16 01:09:20 2016

Only paint the background onto the scrolling contents layer if we have a scrolling contents layer.

BUG= 647169 , 645949 
TEST=CompositedLayerMappingTest.BackgroundPaintedIntoGraphicsLayerIfNotCompositedScrolling
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2

Review-Url: https://codereview.chromium.org/2346883003
Cr-Commit-Position: refs/heads/master@{#419064}

[modify] https://crrev.com/dd59f478410b76764e227fe24642bae6f7b06b26/third_party/WebKit/Source/core/layout/LayoutBoxModelObject.cpp
[modify] https://crrev.com/dd59f478410b76764e227fe24642bae6f7b06b26/third_party/WebKit/Source/core/layout/compositing/CompositedLayerMapping.cpp
[modify] https://crrev.com/dd59f478410b76764e227fe24642bae6f7b06b26/third_party/WebKit/Source/core/layout/compositing/CompositedLayerMappingTest.cpp
[modify] https://crrev.com/dd59f478410b76764e227fe24642bae6f7b06b26/third_party/WebKit/Source/core/paint/PaintLayer.cpp
[modify] https://crrev.com/dd59f478410b76764e227fe24642bae6f7b06b26/third_party/WebKit/Source/core/paint/PaintLayer.h
[modify] https://crrev.com/dd59f478410b76764e227fe24642bae6f7b06b26/third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp
[modify] https://crrev.com/dd59f478410b76764e227fe24642bae6f7b06b26/third_party/WebKit/Source/core/paint/PaintLayerScrollableAreaTest.cpp
[modify] https://crrev.com/dd59f478410b76764e227fe24642bae6f7b06b26/third_party/WebKit/Source/core/paint/PaintLayerScrollableAreaTest.h

Note:Above issue is reproducible on latest canary version i.e 55.0.2862.0

Unfortunately 55.0.2862.0 was built one revision before my fix at #419063. I have confirmed a tip of tree dev build works, but we can wait for the next canary and verify then.

Thanks for the quick turnaround. Please request a merge to M54 once its verified. Next M-54 RC cut is planned at 4 pm PST - 09/20 ( Tuesday) please plan accordingly.

Requesting adharap@ to verify in the latest canary.

I've verified that it is working correctly on Win7 55.0.2866.0 (Official Build) canary (64-bit).

Your change meets the bar and is auto-approved for M54 (branch: 2840)

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/a88c6ad9ff6b006f496679107d484df9ca4782c6

commit a88c6ad9ff6b006f496679107d484df9ca4782c6
Author: Robert Flack <flackr@chromium.org>
Date: Tue Sep 20 20:59:50 2016

Only paint the background onto the scrolling contents layer if we have a scrolling contents layer.

TBR=chrishtr@chromium.org
BUG= 647169 , 645949 
TEST=CompositedLayerMappingTest.BackgroundPaintedIntoGraphicsLayerIfNotCompositedScrolling
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2

Review-Url: https://codereview.chromium.org/2346883003
Cr-Commit-Position: refs/heads/master@{#419064}
(cherry picked from commit dd59f478410b76764e227fe24642bae6f7b06b26)

Review URL: https://codereview.chromium.org/2353623004 .

Cr-Commit-Position: refs/branch-heads/2840@{#448}
Cr-Branched-From: 1ae106dbab4bddd85132d5b75c670794311f4c57-refs/heads/master@{#414607}

[modify] https://crrev.com/a88c6ad9ff6b006f496679107d484df9ca4782c6/third_party/WebKit/Source/core/layout/compositing/CompositedLayerMapping.cpp
[modify] https://crrev.com/a88c6ad9ff6b006f496679107d484df9ca4782c6/third_party/WebKit/Source/core/layout/compositing/CompositedLayerMapping.h
[modify] https://crrev.com/a88c6ad9ff6b006f496679107d484df9ca4782c6/third_party/WebKit/Source/core/layout/compositing/CompositedLayerMappingTest.cpp

Tested the issue on Chrome Beta# 54.0.2840.34 on Windows, Mac and Linux and is no more reproducible.
Hence adding TE-Verified Labels.
Also adding a screenshot for reference.
Thank You.

Deleted: 647169.png 1.2 MB

	647169.png 1.2 MB View Download

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/a88c6ad9ff6b006f496679107d484df9ca4782c6

commit a88c6ad9ff6b006f496679107d484df9ca4782c6
Author: Robert Flack <flackr@chromium.org>
Date: Tue Sep 20 20:59:50 2016

Only paint the background onto the scrolling contents layer if we have a scrolling contents layer.

TBR=chrishtr@chromium.org
BUG= 647169 , 645949 
TEST=CompositedLayerMappingTest.BackgroundPaintedIntoGraphicsLayerIfNotCompositedScrolling
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2

Review-Url: https://codereview.chromium.org/2346883003
Cr-Commit-Position: refs/heads/master@{#419064}
(cherry picked from commit dd59f478410b76764e227fe24642bae6f7b06b26)

Review URL: https://codereview.chromium.org/2353623004 .

Cr-Commit-Position: refs/branch-heads/2840@{#448}
Cr-Branched-From: 1ae106dbab4bddd85132d5b75c670794311f4c57-refs/heads/master@{#414607}

[modify] https://crrev.com/a88c6ad9ff6b006f496679107d484df9ca4782c6/third_party/WebKit/Source/core/layout/compositing/CompositedLayerMapping.cpp
[modify] https://crrev.com/a88c6ad9ff6b006f496679107d484df9ca4782c6/third_party/WebKit/Source/core/layout/compositing/CompositedLayerMapping.h
[modify] https://crrev.com/a88c6ad9ff6b006f496679107d484df9ca4782c6/third_party/WebKit/Source/core/layout/compositing/CompositedLayerMappingTest.cpp