I've complained occasionally about the state my images can get into  
where there are frequent excruciating waits while each SystemWindow  
redraws itself many times slower than normal - these are often  
triggered by debuggers popping up but I also have images where just  
opening a new browser window can sometimes set it off.  (See http:// 
macos.tuwien.ac.at:9009/482103917.asHtml for one discussion about the  
problem).  I can now characterize it a little better, I think: what's  
happening is that the deferred updates flag is getting set to false,  
and yet the WorldState still has a canvas directly on the Display  
form.  If I disable deferred updating, and nil out the canvas on  
WorldState, then I'm pretty sure the problem goes away (and I don't  
notice any particular change in UI responsiveness, though I realize  
there must be some extra copying going on).

Does this ring any bells for anyone?

Avi