Subject: bug#34080: 27.0.50, 26.1; undocumented key bindings for *vc-diff*

From: Stefan Monnier <[email-masked]>
Date: Wed, 09 Jul 2025 17:47:05 -0400

> - that diff-mode-shared-map is the parent of diff-minor-mode-map, not
>   diff-mode-map, d'oh
> - that diff-minor-mode is the mechanism by which the keys are made
>   available when the buffer is read only.
>
> What do you think about replacing activating a minor mode with just
> switching the major mode map?

Fine by me.

> So far as I can tell the disadvantage would be repeating keymap
> definitions, and the advantage would be more standardised
> *Help* output.

We definitely do not want to repeat keymap definitions (users may have
their own and may want to change the ones we provide).

But you can do things like

    (use-local-map (make-composed-map diff-mode-shared-map diff-mode-map))

- Stefan