I'm using Falkon 3.0.0 under Gentoo Linux. QtWebEngine 5.9.4. I'm not using KDE; I run the i3 window manager instead. Steps to reproduce this issue: 1. Open Falkon on workspace 1 2. Visit https://www.electroradio.fm/ 3. Press F11 (fullscreen) 4. Switch to another workspace 5. Return to workspace 1 After the previous steps Falkon becomes unresponsive and displays a blank window. But it still manages to play music somehow. At this point I have to kill Falkon manually and restart it again. Maybe the problem is due to the Intel video drivers, but I'm not sure.
That's bug in i3, other window gets focus (either invalid certificate dialog or popup window with radio player). You can still leave fullscreen with i3 shortcut.