QUESTION
 How can I fix a window's position on the screen without changing
 its position on the Composition Editor?

ANSWER
 Think of the free-form surface of the composition editor as your
 display; so if your window is at the top left of the free form
 surface, it will be at the top left of your display.

 Assuming that you want to place it specifically at test/runtime
 (and not effect its position in the composition editor), you
 could connect from aboutToOpenWidget of the window to a script
 in VisualAge like the following:

 (self subpartNamed: 'Window') yourself primaryWidget
    x: 200;
    y: 400.