Weird behavior on MAC

I’m running a script that continuously draws a line (drawing API).
the script is in a onEnterframe event.
For some reason, and only on a MAC, when the browser loses focus and you pull up another program window in front, then you come back to the flash movie, it somehow kept calculating and displays a full block of red (my lineTo style is red) as if filling up a portion of the stage with lines everywhere (look more like a fill).
I can’t find anything on stage or MC focus. i.e is there a way of executing the enterframe only when the stage has focus? Javascript focus would not work I think.
Any ideas?