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And when you do resize the window layout sucks, automatic layout seems to 
be buggy here (although I must admit that I haven't seen IDLE on other 
platforms).

I have no idea what is causing this bug. Upgrading the Tk version to the very 
latest instead of using the system version doesn't help.

I've checked in a workaround in revision 47091. That patch avoids explicitly 
sizing the stackviewer widget on OSX, which seems to fix this problem. The 
debugger window now is rather less wide than it was before, which means 
users will likely have to resize the window if they want to use the debugger.

I'm leaving this bug open and unassign it in the hope that someone who 
actually knows Tkinter proposed a better fix.