The key you just pressed is not recognized by SDL. To help get this fixed, please report this to the SDL forums/mailing list https://discourse.libsdl.org/ X11 KeyCode 204 (196), X11 KeySym 0x0 ((null)).
I am using TigerVNC to access a remote computer. I can reproduce this consistently by pressing Right ALT and then pressing Left ALT + SHIFT (which will generate this message). This seems to cause Left ALT to stop working in combination with the SHIFT key (and another key to generate a key combo like Alt+Shift+a). It’s possible this is a bug in TigerVNC that is causing it to throw invalid keycodes at you guys.
It appears that even though I am pressing these buttons inside the SDL program, the terminal it is running in gets corrupted. Launching the same program from the same terminal causes the same bug to immediately present itself without me needing to press Right ALT. Launching from a new terminal doesn’t cause this bug to be presented.
apt lists the version of SDL as libsdl2-2.0.0.