What steps will reproduce the problem?
 (1) Setup a chromebook to have two users setup on it.
 (2) Open the browser on the first user, and maximize it.
 (3) Switch to other user, open a browser, and maximize it.
 (4) Switch back to user one.
 (5) Switch back to user two.

What is the expected result?
Shelf should be black in both cases.

What happens instead?
At either step (4) or (5), the shelf is transparent. (If you have a black wallpaper this bug will be hidden presumably).