Here am I looking at the different tcell.Key constants and typing different key combinations in the terminal to see the generated tcell.EventKeys in the debug log. Until I pressed Ctrl+Alt+Backspace… :-D Yep, suddenly there went my X…

So far, it appears as if I can have either only Ctrl or Alt as modifiers. But not in combination. And Shift is just never ever set at all. Interesting.