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A couple useful debugging commands for X11:
`xeyes` will render two eyeballs that follow your mouse cursor. Though I've used it more for entertainment, testing X11 forwarding, and testing configs with `startx`.
`xev` outputs useful information about X11 events, including keypresses and mouse movements. It's useful for discovering the names/codes of keys for keybindings. And running it can even help you recover if some program has captured X11 input and hasn't released it.
And I can't even recall the number of times I ran `glxgears` in my youth to profile and debug video driver issues.