Historically these weren't "edge cases"; all windowed environments sent a "redraw yourself" event to a window when it needed to be shown after having been covered up, that was handled specially by the application. In X11 this was an Expose event, in Windows this was called WM_PAINT. If you had a fancy X server back in the day, you could enable backing store; this kept a framebuffer of the window contents in memory even if the window itself was covered up that could be quickly restored upon exposure. This was incredibly useful and made redraws fast and glitch-free, but back in the days when RAM was expensive (you know, unlike today...) it could not be relied on in all X implementations. Compositing WMs have the equivalent of backing store for all windows, all the time.