[FernetMenta]

We will definitely consider this for Wayland that will become new reference platform.

[Plowhead]

I was using my keyboard and found that the backslash changed this option in win10 from windowed full screen to true fullscreen

[WTCHR]

My issue
Changing the volume in KODI always lead to a black flashing screen when the OSD appeared and disappeared.

Origin
I first used KODI on Ubuntu with the out-of-the box graphics drivers and the cheapest passive cooling NVIDIA graphics card capable of displaying 4K. Playback worked well with 1080p movies (without black flashing when changing the volume) on an external TV with HDMI, but as soon as 4K movies were involved, performance dropped. Therefore, I installed proprietary NVIDIA drivers and there it began with the black flashing when the OSD appeared and disappeared.

Having observed this flashing issue on Windows already when gaming with full screen display and switching between apps, I knew I had to switch to windowed full screen with KODI.

Solution for Ubuntu 20.04.x
Hide bothering items from the OS:

Activate two Gnome extensions:

1. Hide Top Bar by tuxor1337: This will hide the Ubuntu top bar (display it again temporarily with SUPER key)
   
2. GTK Title Bar by velitasali: This will hide all non-GTK application title bar
   

And of course auto-hide the Ubuntu dock from the Display settings.