Hi Ivan and Alan!
I'm not sure that is the correct place for the patch.
Could you enable NSTRACE (by uncommenting the line defining NSTRACE_ENABLED in nsterm.h) and see which function calls `drawRect` (if possible). If that is in a function that is part of Emacs, it's better to check that the frame is visible higher up in the call chain.
You will have to start Emacs from a terminal window in order to see the trace output. You can do this by running "./nextstep/Emacs.app/Content/MacOS/Emacs".