@neauoire I only ever wrote 1 X11 program, but it was using XLib and none of the new-fangled framework nonsense (it was on a DEC Ultrix machine in about 1994). I _think_ you "simply" call XNextEvent; see how xev.c does it https://gitlab.freedesktop.org/xorg/app/xev/-/blob/master/xev.c#L1479

I _think_ by default an X11 client gets all events (for all windows!) unless it has requested only events for particular windows (like their own, which most apps would do); i think that's XSelectInput.