A peculiarity of the X Window System: windows all the way down

Every window system has windows, as an entity. Usually we think of these as being used for, well, windows and window like things; application windows, those extremely annoying pop-up modal dialogs that are always interrupting you at the wrong time, even perhaps things like pop-up menus. In its original state, X has more windows than that. Part of how and why it does this is that X allows windows to nest inside each other, in a window tree, which you can still see today with ‘xwininfo -root -tree‘.

One of the reasons that X has copious nested windows is that X was designed with a particular model of writing X programs in mind, and that model made everything into a (nested) window. Seriously, everything. In an old fashioned X application, windows are everywhere. Buttons are windows (or several windows if they’re radio buttons or the like), text areas are windows, menu entries are each a window of their own within the window that is the menu, visible containers of things are windows (with more windows nested inside them), and so on.
↫ Chris Siebenmann

This is wild.