Problem: I experienced Spotify being stuck in full screen mode. The title bar with minimize, maximize and close wasn't present and no keyboard shortcut I know about would exit full screen.
@mastier has improved the solution - see here
Works both in 18.04.01 and 20.04.x
Source: https://community.spotify.com/t5/Desktop-Linux/Full-screen-under-Ubuntu-18-04/m-p/4546981/highlight/true#M16717 (Even though this didn't fix the problem for the author, it worked for me)