Chrome Version       : 69.0.3464.0
OS Version: Linux

What steps will reproduce the problem?
1. Open video in PIP
2. Resize until letterboxing appears
3. Unfocus window

What is the expected result?
All the buttons hide.

What happens instead of that?
The close button is still visible.

In a debug build, the letter boxing is blue so I would guess that the issue is that we are missing a background colour or something and in a release build it just shows whatever was there. It will most likely be fixed when we will no longer allow letter boxing but filing in case of.