Confirmed in ToT; Goobuntu. Sending over to paint team.

Using the per-revision bisect providing the bisect results,
Good build:55.0.2872.0(Revision: 420859).
Bad build: 55.0.2873.0 (Revision:421052).

You are probably looking for a change made after 420954 (known good), but no later than 420955 (first known bad).

CHANGELOG URL:
-----------------   https://chromium.googlesource.com/chromium/src/+log/38d12538c8af0b8afbf9c5b586b10902b9ba1303..ef06887f125941e96ab19fba834d9027d36d4c88

From the CL above, assigning the issue to the concern owner

@schenney - 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.

Review-Url: https://codereview.chromium.org/2358203002

This is a recent regression seen on M55 adding stable blocker, Please feel free to edit or remove if this is not the case.

Thanks!

schenney@: Could you please take a look at this.

Yes, I have a bunch of similar issues related to the changes to compositing scrollers.

It seems we are one pixel too small on the painted layer, leaving a gap to the scrollbar painting. On the crbug drop-down bug, we are one pixel too big, I think, obscuring the border. Looks like pixel snapping problems or sub-pixel error accumulation problems.

We are snapping the layer size down when in fact we should be snapping up. In  crbug.com/651305  we are too short and here we are too narrow.

Not a duplicate after all. Fixing this does not fix the problem with crbug.com.

Just to update the latest behavior of the bug, Issue is still repro on chrome latest canary M56-56.0.2891.0.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/6c715c7a413e006594b26a2b59783d2d9d01f311

commit 6c715c7a413e006594b26a2b59783d2d9d01f311
Author: schenney <schenney@chromium.org>
Date: Mon Oct 17 18:33:35 2016

Use the pixelSnappedScrollWidth/Height for contentsSize

PaintLayerScrollableArea::contentsSize was using the LayoutUnit::toInt
method to convert to int for the size. However, when sizing a Graphicslayer
from this value, it results in layers that are too narrow in some cases,
giving single pixel gaps between the scrolling content and the scrollbars.

The correct value is the pixelSnappedScrollWidth, that accounts for
sub-pixel position of surrounding elements.

Adds a test that fails in m55 and passes with the change, derived from the
site that the bug was reported for.

R=flackr@chromium.org
BUG= 653847 
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2

Review-Url: https://codereview.chromium.org/2416453006
Cr-Commit-Position: refs/heads/master@{#425727}

[modify] https://crrev.com/6c715c7a413e006594b26a2b59783d2d9d01f311/third_party/WebKit/LayoutTests/FlagExpectations/enable-slimming-paint-v2
[add] https://crrev.com/6c715c7a413e006594b26a2b59783d2d9d01f311/third_party/WebKit/LayoutTests/compositing/overflow/fractional-sized-scrolling-layer-expected.png
[add] https://crrev.com/6c715c7a413e006594b26a2b59783d2d9d01f311/third_party/WebKit/LayoutTests/compositing/overflow/fractional-sized-scrolling-layer-expected.txt
[add] https://crrev.com/6c715c7a413e006594b26a2b59783d2d9d01f311/third_party/WebKit/LayoutTests/compositing/overflow/fractional-sized-scrolling-layer.html
[add] https://crrev.com/6c715c7a413e006594b26a2b59783d2d9d01f311/third_party/WebKit/LayoutTests/compositing/overflow/scaled-overflow-expected.png
[add] https://crrev.com/6c715c7a413e006594b26a2b59783d2d9d01f311/third_party/WebKit/LayoutTests/compositing/overflow/scaled-overflow-expected.txt
[add] https://crrev.com/6c715c7a413e006594b26a2b59783d2d9d01f311/third_party/WebKit/LayoutTests/compositing/overflow/scaled-overflow.html
[modify] https://crrev.com/6c715c7a413e006594b26a2b59783d2d9d01f311/third_party/WebKit/LayoutTests/platform/mac-mac10.9/paint/invalidation/invalidate-caret-in-composited-scrolling-container-expected.txt
[modify] https://crrev.com/6c715c7a413e006594b26a2b59783d2d9d01f311/third_party/WebKit/LayoutTests/platform/mac-mac10.9/paint/invalidation/selection/selection-in-composited-scrolling-container-expected.txt
[modify] https://crrev.com/6c715c7a413e006594b26a2b59783d2d9d01f311/third_party/WebKit/LayoutTests/platform/mac/paint/invalidation/invalidate-caret-in-composited-scrolling-container-expected.txt
[modify] https://crrev.com/6c715c7a413e006594b26a2b59783d2d9d01f311/third_party/WebKit/LayoutTests/platform/mac/paint/invalidation/selection/selection-in-composited-scrolling-container-expected.txt
[modify] https://crrev.com/6c715c7a413e006594b26a2b59783d2d9d01f311/third_party/WebKit/LayoutTests/platform/win/fast/overflow/overflow-rtl-expected.png
[modify] https://crrev.com/6c715c7a413e006594b26a2b59783d2d9d01f311/third_party/WebKit/LayoutTests/platform/win/fast/overflow/overflow-rtl-vertical-expected.png
[add] https://crrev.com/6c715c7a413e006594b26a2b59783d2d9d01f311/third_party/WebKit/LayoutTests/virtual/prefer_compositing_to_lcd_text/compositing/overflow/fractional-sized-scrolling-layer-expected.png
[add] https://crrev.com/6c715c7a413e006594b26a2b59783d2d9d01f311/third_party/WebKit/LayoutTests/virtual/prefer_compositing_to_lcd_text/compositing/overflow/fractional-sized-scrolling-layer-expected.txt
[add] https://crrev.com/6c715c7a413e006594b26a2b59783d2d9d01f311/third_party/WebKit/LayoutTests/virtual/prefer_compositing_to_lcd_text/compositing/overflow/scaled-overflow-expected.png
[add] https://crrev.com/6c715c7a413e006594b26a2b59783d2d9d01f311/third_party/WebKit/LayoutTests/virtual/prefer_compositing_to_lcd_text/compositing/overflow/scaled-overflow-expected.txt
[modify] https://crrev.com/6c715c7a413e006594b26a2b59783d2d9d01f311/third_party/WebKit/Source/core/layout/compositing/CompositedLayerMapping.cpp
[modify] https://crrev.com/6c715c7a413e006594b26a2b59783d2d9d01f311/third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp

Rechecked this on Windows 10 for 56.0.2896.0 and on MAC 10.11.6, Ubuntu 14.04 for chrome version 56.0.2896.0. Fix is working as intended. No blue line is displayed in the pull down menu.

Adding TE-Verified labels.

Deleted: Screen Shot 2016-10-20 at 2.05.54 PM.png 124 KB

	Screen Shot 2016-10-20 at 2.05.54 PM.png 124 KB View Download

 Issue 665619  has been merged into this issue.