ovanieva@, this behavior also exist before I changed the UI. When we turn on mirror mode, there's a black out, after which the display is refreshed and the shadow is shown on the button that has the focus.
When we toggle other toggle button, there's no such black out, so the display will be not refreshed, so the shadow will not be shown.

Deleted: 3669994040298161570.jpg 26.8 KB

	3669994040298161570.jpg 26.8 KB View Download

I see - you are right. Let's remove the shadow anyhow as current behavior is distracting. 

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/3a4ec7c2070d898b86bd96313de263179c05c9c1

commit 3a4ec7c2070d898b86bd96313de263179c05c9c1
Author: Weidong Guo <weidongg@chromium.org>
Date: Wed Nov 22 20:09:40 2017

Remove the shadow on the checkbox when mirror mode is turned on

Background:
When the user check the 'Mirror Internal Display' checkbox, there will
be a fade-in animation for the entire display, after which the setting
window will be focused and, thus, the focused checkbox will show a
shadow.

Changes:
Prevent the shadow by blurring the checkbox when it is tapped.

Bug:  785070 
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: I2581483b6fe8cd11386e0363a7e32508560652c3
Reviewed-on: https://chromium-review.googlesource.com/783630
Commit-Queue: Weidong Guo <weidongg@chromium.org>
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#518724}
[modify] https://crrev.com/3a4ec7c2070d898b86bd96313de263179c05c9c1/chrome/browser/resources/settings/device_page/display.js
[modify] https://crrev.com/3a4ec7c2070d898b86bd96313de263179c05c9c1/chrome/test/data/webui/settings/device_page_tests.js