Thanks for the comments.
1. I previously said somewhere 'enabled but initially off' by default. I intend to make sure of that before committing.
2. Option menu sounds good..
3. I tried out a *much* lighter gray, as with Notepad++. Saimadhav is leaving that to me to determin and change.
4. If inactiveselectbackground is needed and there is no alternative (Saimadhav?) I would be inclined to skip pre-8.5.
5. I have put off writing a doc until the spec is finished. (And I would like to have help.txt auto-generated from idle.rst, which is an issue somewhere.)
6. I do not intend line numbering for Shell. Rather, the sidebar should be used for prompt and output markers to solve #7676. The patch has been written with this in mind. How to handle this vis-a-vis config-extensions had not been decided yet. I would prefer one [sidebar] section, but Saimadhav does not think that is possible. In any case, we have focused first on just line numbering, which I would like to push with the revisions indicated above, and then adding breakpoint toggling (in a followup patch, separately reviewed) and the shell sidebar (in yet another patch, ditto).
(Note. see #22209)