Todd, the LineNumbers.py extension from the IdleX project contains 
work-arounds for interacting cleanly with the Code Context extension. It 
also has a hack for dealing with the shortcomings of the 
Percolator/Delegator ordering.

There are other shortcomings in the extension that I could not address 
from IDLE's extension loading architecture. Notably, IDLE does not have 
a method to broadcast that a font was changed, so the extension polls 
every second to see what the font is. The Code Context extension also 
does this. This is clearly an inefficient way to handle identifying a 
configuration change.

I released IdleX with the University of Illinois. Presently, the code is 
under the NCSA license agreement and assigned to the Board of Trustees. 
If the PSF would allow inclusion as-is, then I'm all for it. Otherwise I 
will need to ask UIUC about how to re-license the code.