Spent some time today with IDLE and PDB, and I've ruled out any other program on my system as the culprit:

- I can create a tkinter GUI that will respond to a binding like <Control-Key-x><Control-Key-s> and it works fine.
- In pdb, I can bind <<do-nothing>> to a callback and it is getting fired when Ctrl-x is pressed.  Thus, I was wrong to think anything else is "eating" the ctrl-x.
- Also in pdb, an .event_info('<<save-window>>') call on the editor's Text widget returns ('<Control-Key-x><Control-Key-s>',), so it looks bound correctly?  If I generate a <<save-window>> event, the dialog pops up as it should.

Nevertheless, Ctrl-x Ctrl-s opens the find dialog, and no other Ctrl-x (other keystroke) binding works.  I'm stumped.