[toast0]

> in this case it is better to use GTK as backend instead of targeting X11 directly that may even be replaced with Wayland on major Linux distribtions;

X11 can work on Windows (many Xservers available, since forever), MacOS (used to ship with one, but now you've got to install one), and you can run an Xwayland in Wayland. So it's as cross platform as you can get. Might not look great, but then cross platform doesn't usually look great anyway.

If you're going to do X, avoid Xlib, it adds restrictive abstractions on top of X protocol and really confuses things. XCB is much closer to just reasonable interfaces to the protocol. Ultimately, X11 is a distributed systems communication protocol which happens to have graphical output as a side effect; understanding the communications part first lets you get the most out of it.