[Bug 1812394] [NEW] Video playback faintly shows applications behind active browser window

[Leonardus]

Public bug reported:

Ubuntu desktop 18.10, Mozilla Firefox 64.0

To reproduce:
1. Open any application (such as Nautilus file manager, or the terminal emulator)
2. Open Firefox above this application
3. Begin video playback (I've found that it's easier to notice this bug when the video is a dark gray or black. This video worked well: < https://youtu.be/jYFk1O_t43A >)

What happens:
In the region where the video is playing, you will faintly see the application(s) that are behind Firefox.

What should happen:
The video should completely obscure any applications that are not above the browser window.

I cannot reliably reproduce this bug when the player is in full screen -
i.e. it must be in theater mode or normal player.

** Affects: firefox (Ubuntu)
     Importance: Undecided
         Status: Confirmed

** Attachment added: "A screenshot of the bug"
   https://bugs.launchpad.net/bugs/1812394/+attachment/5230290/+files/hhcmiLh.png

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Title:
  Video playback faintly shows applications behind active browser window

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