For a Simple Refresh window, only those pixels actually on-screen are maintained by the system. When part of a Simple Refresh window is obscured, the imagery that was there is lost. As well, any rendering into obscured portions of such a window is discarded. When part of the window is newly revealed (either because the window was just made larger, or because that part used to be obscured by another window), the application must refresh any rendering it wishes to appear into that part. The application will learn that refresh is needed because Intuition sends an IDCMP_REFRESHWINDOW event.