A more than minimal patch has been merged.  Further work will be on new issues.  Remaining problems not previously discussed:

1. Tab focus traversal is wrong.  This is generic to Query subclasses that add widgets.  See #37325.

2. When the customize box is cancelled, focus goes to the Shell, even when I add 'event.widget.focus_set/force()'.  When the parent of the box was the editor, 'self.shell.text.focus_set' did not work to shift it to the Shell text.

Does anyone else have any idea how to put the focus back on the editor after cancel?