Popups being regular items drawn inside the window, instead of their own windows, makes them useless on desktop platforms.

E.g. if you make a windowed UI with popup menus aligned along the bottom (for example, a "Zoom level" combobox for a viewer program), or a combox in a small window, the menu won't be visible/usable because it can't go outside the window.

This is, quite frankly, insane.