but if you stop rendering in a visible (only out of focus) window, the contents get overwritten (at least in windows) and that looks pretty ugly... so if only focus information is available, I'll still have to refresh the image when paused. even if the window is iconified and it doesn't matter...
EDIT:
if it helps, the X11 events should be MapNotify/UnmapNotify and the Win32 event WM_ACTIVATE, couldn't find anything for OSX, but I'm sure there is such an event.