what part of X maps screen coordinates to objects?

[George Nychis]

I am trying to gain a very technical understanding of how X maps
coordinates on the 2d screen to objects.  For example, there are many
applications running with graphical interfaces to them active on the
screen, and each application interface has dozens (if not hundreds) of
objects.  Some objects are contained within other objects.

So, what I am trying to learn more about is how X maps a screen coordinate
(and an action like a mouse click) to an object quickly.  In particular,
when there are objects within objects, how it narrows the search down to
the "smallest" element that your mouse is hovering over when a click event
fired.

I'd love to know about what data structures and algorithms are used to
perform this mapping between the 2D graphical space to objects.  Pointers
to prior discussion, documentation, code... anything is helpful.

Thanks much!

- George
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