Version: 52 and 53
OS: Chrome

What steps will reproduce the problem?
(1) Adjust scale factor to 1.25
(2) Open a chrome window pointed to NTP or other page with light background.
(3) Open an incognito window
(4) Position the windows with the incognito window on top and its right edge over the lighter window.
(5) Click and drag the right edge of the incognito window to resize it several times repeatedly.

What do you see?
A small (1px?) gap appears between the window chrome and the window shadow where you can see through to the layer underneath.

What is the expected output?
No gap.