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.