Might have been technical limitations. When you allow overlap, you can’t use the screen as _the_ backing store for pixels anymore, and either memory usage goes

Might have been technical limitations. When you allow overlap, you can’t use the screen as _the_ backing store for pixels anymore, and either memory usage goes up (a lot, if you’ve several large windows open), or you have to invent redraw events (which may be slow even for relatively simple content, if you have to swap in the drawing code). If you keep the screen buffer as (part of) the backing store for the pixels of windows, you also have to make drawing code clip to any overlapping windows. If you don’t, memory usage goes up again. And memory could be tight. As an extreme example, the screen memory on an original Mac was about the same size as the amount of memory available to the running application.