Correction:
Version: 54.0.2825.0 Dev

Attaching screen-cast for reference.

Deleted: Actual_Sharescreen.ogv 2.2 MB

	Actual_Sharescreen.ogv 2.2 MB View Download

Deleted: Scrollbar.png 217 KB

	Scrollbar.png 217 KB View Download

Able to reproduce the issue on Windows-10 using chrome latest Dev M54-54.0.2825.0. Observed extra vertical scrollbar in the sharing screen.

Note: This issue is not seen on Mac OS 10.11.6

I think it is due to the interaction between TabbedPane and ScrollView.
This is what I found when the ScrollView is in a tab of TabbedPane:
1. In [1], vp_size.width() is always content_size.width() - 2, regardless of what you return from content view's GetPreferredSize.
2. The logic of [2] is not quite right, when |hide_horizontal_scrollbar_| is true. Because when |hide_horizontal_scrollbar_| is true, there is no need to deduct the bar height from vp_size.height() for comparison.

msw@, can you take a look? Or do you know whom to refer to?

[1]https://cs.chromium.org/chromium/src/ui/views/controls/scroll_view.cc?q=scrollView::compu&sq=package:chromium&l=590

[2]https://cs.chromium.org/chromium/src/ui/views/controls/scroll_view.cc?q=scrollView::compu&sq=package:chromium&l=603

I don't know that code well offhand; did you try using git blame?

Update:
In [1], it set content_width to be the width of the ScrollView control.
In [2], it set viewport_bounds.width to be the content_width - 2 * border width. (Look into the implementation of GetContentBounds)

Then if in [3], the content view's Layout() function does not SetSize explicitly, a horizontal bar will be always visible, because the viewport_bounds.width < content_width.

Then look at [4], we compared |content_height| with |viewport_bounds.height - scroll_bar.height| to determine whether displaying vertical bar. So if the content just fill the container exactly, a vertical bar will be visible.


[1]https://cs.chromium.org/chromium/src/ui/views/controls/scroll_view.cc?q=scrollview&sq=package:chromium&l=298
[2]https://cs.chromium.org/chromium/src/ui/views/controls/scroll_view.cc?q=scrollview&sq=package:chromium&l=316
[3]https://cs.chromium.org/chromium/src/ui/views/controls/scroll_view.cc?q=scrollview&sq=package:chromium&l=343
[4]https://cs.chromium.org/chromium/src/ui/views/controls/scroll_view.cc?sq=package:chromium&rcl=1472733616&l=603

I think a thorough fix should be
1. In [1], we should Inset the ScrollView Bound with its border, and assign it to content_width.
2. In [4], we should consider the case when |hide_horizontal_scrollbar_| is true, and in this case we should never offset the viewport_bounds.height with scrollbar height for comparison.

[1][4] Look at #5 for code search link.

sky@, can you take a look?

You are one of the major authors of scroll_view.cc

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/6d191b6ae63639d0c55046ef4bc909726d25a30c

commit 6d191b6ae63639d0c55046ef4bc909726d25a30c
Author: qiangchen <qiangchen@chromium.org>
Date: Tue Sep 06 23:17:17 2016

Bug Fix: Desktop Picker Window Shows Unnecessary Scrollbar

When the number of source items just exactly fill the container,
we displayed the scroll bar.

In this CL, we fixed the issue, and not displayed scroll bar in
that case.

BUG= 636338 

Review-Url: https://codereview.chromium.org/2304743002
Cr-Commit-Position: refs/heads/master@{#416763}

[modify] https://crrev.com/6d191b6ae63639d0c55046ef4bc909726d25a30c/chrome/browser/ui/views/desktop_capture/desktop_media_list_view.cc

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/+/5a81ddeed15a5a16a1142f8fa3cae97dc3099ddb

commit 5a81ddeed15a5a16a1142f8fa3cae97dc3099ddb
Author: qiangchen <qiangchen@chromium.org>
Date: Wed Sep 07 22:47:47 2016

Bug Fix: Desktop Picker Window Shows Unnecessary Scrollbar [M54]

When the number of source items just exactly fill the container,
we displayed the scroll bar.

In this CL, we fixed the issue, and not displayed scroll bar in
that case.

BUG= 636338 

TBR=msw@chromium.org,sky@chromium.org
NOPRESUBMIT=TRUE
NOTRY=TRUE

Review-Url: https://codereview.chromium.org/2304743002
Cr-Commit-Position: refs/heads/master@{#416763}
(cherry picked from commit 6d191b6ae63639d0c55046ef4bc909726d25a30c)

Review-Url: https://codereview.chromium.org/2324613002
Cr-Commit-Position: refs/branch-heads/2840@{#222}
Cr-Branched-From: 1ae106dbab4bddd85132d5b75c670794311f4c57-refs/heads/master@{#414607}

[modify] https://crrev.com/5a81ddeed15a5a16a1142f8fa3cae97dc3099ddb/chrome/browser/ui/views/desktop_capture/desktop_media_list_view.cc

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/5a81ddeed15a5a16a1142f8fa3cae97dc3099ddb

commit 5a81ddeed15a5a16a1142f8fa3cae97dc3099ddb
Author: qiangchen <qiangchen@chromium.org>
Date: Wed Sep 07 22:47:47 2016

Bug Fix: Desktop Picker Window Shows Unnecessary Scrollbar [M54]

When the number of source items just exactly fill the container,
we displayed the scroll bar.

In this CL, we fixed the issue, and not displayed scroll bar in
that case.

BUG= 636338 

TBR=msw@chromium.org,sky@chromium.org
NOPRESUBMIT=TRUE
NOTRY=TRUE

Review-Url: https://codereview.chromium.org/2304743002
Cr-Commit-Position: refs/heads/master@{#416763}
(cherry picked from commit 6d191b6ae63639d0c55046ef4bc909726d25a30c)

Review-Url: https://codereview.chromium.org/2324613002
Cr-Commit-Position: refs/branch-heads/2840@{#222}
Cr-Branched-From: 1ae106dbab4bddd85132d5b75c670794311f4c57-refs/heads/master@{#414607}

[modify] https://crrev.com/5a81ddeed15a5a16a1142f8fa3cae97dc3099ddb/chrome/browser/ui/views/desktop_capture/desktop_media_list_view.cc