After my change, scrollers get an outline (a focus ring) when being focused. 

rbpotter@, I understand you are work on this UI.

You can remove this blue outline by putting |outline: none| in your CSS.

Confirmed this regression is due to the linked change by testing with --disable-blink-features=KeyboardFocusableScrollers as suggested in the CL description.

Regarding adding outline:none to the CSS, if we simply remove the outline we will have the same bug as https://crbug.com/905999 where focus appears to be lost when tabbing through the page.

Either the scroller needs to have tabIndex=-1, or we need to add something similar to the bookmark page's MouseFocusBehavior to detect whether focus has been moved to the element due to the mouse or the keyboard to apply the correct CSS.

There are likely a lot of similar regressions (either undesired focus outlines, or focus appearing to be lost) across all of Web UI; I discovered a few more just in Print Preview while doing some brief testing. For each regression, we will need to determine which of these approaches should be used, or, if neither is feasible, as may be the case for the history page based on bug 905999, find some alternative solution.

M-72 branch cut is in less than two weeks, and next week is a short week in the US. Is there any chance we could consider punting this change to M-73, to allow more time to find and fix all the Web UI regressions?

> Either the scroller needs to have tabIndex=-1, ...
Sounds like a good enough workaround until you've adapted the Custom Element?

> Is there any chance we could consider punting this change to M-73, to allow more time to find and fix all the Web UI regressions?
I'm not against that but I guess it would be good to let it stay enabled a little longer so we can collect more bugs reports. So far I've seen no bugs on external web content.

It's not a custom element, the scroller is a <div> in this case, but there is a different regression in Print Preview where we do have a custom element that cannot be taken out of the tab order as you suggest.

Since the recommended fix is just to explicitly remove all scrollers from the tab order, will land a CL doing that for Print Preview in the places where it is possible to do so.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/d12f01a66603de0606db5544cc17b76ea12188d9

commit d12f01a66603de0606db5544cc17b76ea12188d9
Author: rbpotter <rbpotter@chromium.org>
Date: Mon Nov 19 23:18:41 2018

Fix Print Preview sidebar outline

The scroller is now focusable due to Blink changes at r608013.
Remove the outline and take it out of the tab order.

Bug:  905604 
Change-Id: Ib8154c691e1240b27118288950c8efdf46c803b6
Reviewed-on: https://chromium-review.googlesource.com/c/1343285
Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Commit-Queue: Rebekah Potter <rbpotter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#609495}
[modify] https://crrev.com/d12f01a66603de0606db5544cc17b76ea12188d9/chrome/browser/resources/print_preview/new/app.html

Update: 
Rechecked the above issue on canary build #72.0.3616.0 using Windows(7,8,8.1,10) ,Mac(10.13.1 ,10.13.6 ,10.14.2) and Linux(14.04 LTS) and it is fixed.

Deleted: Fixed_behaviour.mov 1.5 MB

	Fixed_behaviour.mov 1.5 MB View Download

Closing this issue since it is now fixed. Opened additional bugs for some other issues in Print Preview at https://crbug.com/906758 and https://crbug.com/906760

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/38d4901b7a0ab2cd4de844c5ea9e0c99ac54bce4

commit 38d4901b7a0ab2cd4de844c5ea9e0c99ac54bce4
Author: Rebekah Potter <rbpotter@chromium.org>
Date: Wed Nov 21 19:53:45 2018

Revert "Fix Print Preview sidebar outline"

This reverts commit d12f01a66603de0606db5544cc17b76ea12188d9.

Reason for revert: Causes crbug.com/907378, and the Blink change that
caused that bug is expected to be reverted.

Original change's description:
> Fix Print Preview sidebar outline
> 
> The scroller is now focusable due to Blink changes at r608013.
> Remove the outline and take it out of the tab order.
> 
> Bug:  905604 
> Change-Id: Ib8154c691e1240b27118288950c8efdf46c803b6
> Reviewed-on: https://chromium-review.googlesource.com/c/1343285
> Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
> Commit-Queue: Rebekah Potter <rbpotter@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#609495}

TBR=dpapad@chromium.org,rbpotter@chromium.org

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug:  905604 
Change-Id: Iea4a67f80284091c2acf0fee24215a7f5b174a03
Reviewed-on: https://chromium-review.googlesource.com/c/1344837
Reviewed-by: Rebekah Potter <rbpotter@chromium.org>
Commit-Queue: Rebekah Potter <rbpotter@chromium.org>
Cr-Commit-Position: refs/heads/master@{#610184}
[modify] https://crrev.com/38d4901b7a0ab2cd4de844c5ea9e0c99ac54bce4/chrome/browser/resources/print_preview/new/app.html

Update: 
Rechecked the above issue on canary build #72.0.3618.0 using Windows(7,8,8.1,10) ,Mac(10.13.1 ,10.13.6 ,10.14.2) and Linux(14.04 LTS) and it is fixed.

Deleted: Fixed_Behaviour.mov 1.4 MB

	Fixed_Behaviour.mov 1.4 MB View Download

Reopening this since the fix had to be reverted. This is not an issue currently since the Blink CL has been reverted, but should be tracked so that it can be fixed before the Blink change is re-landed.

Since "KeyboardFocusableScrollers try #2", clicking a scroller will not focus it so you shouldn't see that blue outline after a click.

https://chromium-review.googlesource.com/c/chromium/src/+/1354450

Please verify with --enable-experimental-web-platform-features.