I found that even with the attempt to turn off shortcuts when entering text in textbox, they still had key presses interpreted as the shortcuts, at least with the tkagg backend. I have what appears to be a working solution (store the callback, call self.canvas.callbacks.disconnect(cid) to completely disconnect everything looking for keystrokes in that axes in begin_typing(), then reconnect the callbacks in stop_typing(), but I don't have time to submit pull request right now. This would be my first time submitting to any open source project, much less MPL. Hell, I haven't even used GIT on a project involving more than just myself. I thought it'd be good to throw it out as an issue before I got ahead of myself.

-using tkagg backend
-using latest (yesterday's) master branch to test out TextBox widget.
-using winpython 64bit 3.5.2.3QT5, except for MPL which I built from source