EDCE runs on it own desktop on a Mac. If I leave it running there while I switch to a different desktop to get some work done and then go away the display never turns off and the screensaver cuts in after the preset period. But this can mean that the screen saver runs all night when I'd really rather have the display go dark.

This is easily demonstrated by setting display go dark to 1 minute and screensaver activation time to 2 minutes. Then leaving the machine alone with and without EDCE running in its own desktop.

that's interesting, but I can not explain why the screensaver is not kicking in - I am sure somehow the program is disturbing the peace - could be related to display calls being made to the video card. (a wild guess)...if it was minimized I wonder how it would behave.

Did the experiment. Set screen blackout to 1 minute and screen saver to 2 minutes, normal and expected behaviour is that screen goes dark after 1 minute.

If EDCE is running but not in a game (displaying main menu) then it behaves normally. If a game is active without user interaction whether current or  non current desktop or  minimised, the screensaver displays after 2 minutes.