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[self setNeedDisplay:YES] just send a message to main thread to refresh the view then return, but the main thread is blocked, so it can't work. 

[self displsyIfNeed] just force main thread to refresh the view and won't return till refreshed, so it will block your invoke thread if main thread is blocked. 

[self display] maybe the performance is different on 10.5 and 10.6.