Narrow Bisect Info:
https://chromium.googlesource.com/chromium/src/+log/dc683d2a5c9af8b55245ce5ab2253d0b6977ccfb..911e6e4e3b6b47416dc89c0a61fb585f86bb1afa?pretty=fuller&n=1000

Suspecting:r419381?

Kindly help to reassign if your change is not the cause for this issue.

My change couldn't have caused this.

None of the changes from #1 seem particularly suspicious, so this could use a re-bisect (preferably a per-revision bisect).

Manual Bisect Info:
--------------
Bad build--55.0.2863.0--Revision-419330

Good build-55.0.2864.0--Revision-419385

After executing the per-revision bisect script , i got the following CL's between good and bad build versions

CHANGELOG URL:
----------------
https://chromium.googlesource.com/chromium/src/+log/ba9df880383908b052d5a7d00abb32deec94cfc5..d5631a9bc17ba8454999183de2dd8f08682ca3c3
  
Possible suspect:
----------------
https://chromium.googlesource.com/chromium/src/+/d5631a9bc17ba8454999183de2dd8f08682ca3c3

Review-Url: https://codereview.chromium.org/2344423002

skia-deps-roller@  Could you please look into this issue, if it's related to your change?  if not could you please help us to reassign this issue to the right owner.

Thanks..

Reassigning to a specific developer.

fmalita@  Could you please look into this issue, if it's related to your change?  if not could you please help us to reassign this issue to the right owner.

Thanks..

The Skia CL in c#4 doesn't touch any code used in Chromium, so it cannot be the culprit (inconsistent bisects -> flaky repro?).

I cannot repro on Linux builds (tried ToT, 56.0.2924.59).

Also, that's not a double scrollbar.  The outer widget is a resize dragbar, present in both cases, then there's also an inner scrollbar (also present in both cases).  What seems to happen is the device emulation scrollbar changes appearance from the thin/frameless theme to the heavier version used in other devtools panels.

CCing some DevTools/UI experts.

Ah, I was watching the unmaximized video, sorry.  That is a double scrollbar indeed - looks like we're painting both the thin and the heavy scrollbar widgets someone.

The rest of my comments stands.

From the narrow bisect in Comment #1, the patch to change the behavior of meta viewport changes seems plausible as a cause. Is it?

Could you please re-bisect this? I reverted my change but this still occurs.

Rebisected the issue & please find the below per revision bisect information :

Good build--55.0.2863.0--Revision-419330
Bad build--55.0.2864.0--Revision--419385

Tool Bisect info:
------------------
You are probably looking for a change made after 419377 (known good), but no later than 419378 (first known bad).
CHANGELOG URL:
The script might not always return single CL as suspectas some perf builds might  get missing due to failure.

  https://chromium.googlesource.com/chromium/src/+log/dc683d2a5c9af8b55245ce5ab2253d0b6977ccfb..795773c25806d52ae98b77d2cdd27d0325319b0f

Review-Url: https://codereview.chromium.org/2341843002

pdr@Kindly take a look and please help us to reassign this issue to a right owner if not with respect to this change.

Thanks..

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/cda5197c6532a6ff59171f0bbcb5b34e0652b3db

commit cda5197c6532a6ff59171f0bbcb5b34e0652b3db
Author: pdr <pdr@chromium.org>
Date: Mon Feb 13 21:23:23 2017

Ensure FrameView scrollbars are updated when changing device emulation modes

Device emulation enables visual viewport scrollbar behavior and
we need to ensure the FrameView's scrollbars are removed when
we begin emulating, and restored when we stop emulating.

BUG= 680894 
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2

Review-Url: https://codereview.chromium.org/2695493003
Cr-Commit-Position: refs/heads/master@{#450093}

[modify] https://crrev.com/cda5197c6532a6ff59171f0bbcb5b34e0652b3db/third_party/WebKit/Source/core/frame/FrameView.cpp
[modify] https://crrev.com/cda5197c6532a6ff59171f0bbcb5b34e0652b3db/third_party/WebKit/Source/core/frame/FrameView.h
[modify] https://crrev.com/cda5197c6532a6ff59171f0bbcb5b34e0652b3db/third_party/WebKit/Source/core/frame/VisualViewport.cpp
[modify] https://crrev.com/cda5197c6532a6ff59171f0bbcb5b34e0652b3db/third_party/WebKit/Source/web/tests/WebViewTest.cpp

Tentative request for a merge into M57. There's some risk in merging, and the risk of not merging is small (double scrollbar).

Your change meets the bar and is auto-approved for M57. Please go ahead and merge the CL to branch 2987 manually. Please contact milestone owner if you have questions.
Owners: amineer@(clank), cmasso@(bling), ketakid@(cros), govind@(desktop)

For more details visit https://www.chromium.org/issue-tracking/autotriage - Your friendly Sheriffbot

Per comment #13, there is a some risk in merging and the risk of not merging is small (double scrollbar). Also issue reported here exists from M55 on wards. Can this wait until M58?

Rejecting merge to M57 based on comment #13 and #15.