Thanks for your answer. When you wrote "the basics of X11", did you mean I
should know the overall structure of the X11 protocol? That's going to be
a lot of stuff to read...

However, it seems that I would have to look at the code of Ruby/X11 and
guess and try out how it does what. That's actually what I wanted to
avoid, because I usually struggle enough with my own code, and don't like
time-consuming exercises like that.

Malte