This has been previously found on 2.3/2.4 and verified again with 2.5b1
(was hoping that the 1.2 IDLE update would fix):

When you bring up the debugger window in IDLE on OSX, you'll notice that
it's vibrating, as if it's stuck in a resize loop.  Unless you
immediately resize the window manually, IDLE will hang the next time you
try to do something that updates the debugger window.

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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.

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Please try running IDLE w/o the subprocess by
starting it with the -n switch.  Does the issue
still exist?

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You'd have to test with a version of python before 2.5b1, after that I've added a 
workaround for this issue. This workaround makes the debugger window 
narrower than intended, but at least you have a working version of IDLE.

My guess is that the root cause of this issue is a bug in TkAqua. I haven't filed a 
bug with them because I have never programmed in Tcl and would therefore 
require lots of time to get a Tcl program going that demonstrates this problem.

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Could you revert your fix locally and try running
w/o the subprocess?  If that fixes it you could
blame the rather elaborate communication between
the subprocess and the debugger gui, and it would
be good to check before blaming AquaTk.

If it's still there, maybe you could provide a cutdown
in Tkinter (w/o IDLE) and change the Category to 
Tkinter?

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The vibration problem also occurs if you start idle with 'python idle.py -n'. 

I'm moving this to Tkinter. I'll see if I can extract a small program that shows the 
problem, I'm not exactly a Tkinter expert. I'm also unassigning this bug, there's 
little change that I'm going to fix this.

Is this still an issue with later versions of Python and/or OSX?

The problem appears to be gone in current releases (at least on OSX 10.8 with Tk 8.5) and I therefore propose to close the issue.

Undoubtedly, it was a bug in an earlier version of Aqua Tk; it doesn't appear to occur with current Tk's.