Re: Should `revert-buffer' preserve text-scaling by default?

[Karl Fogel <kfogel@red-bean.com>]

On 29 Nov 2019, Eli Zaretskii wrote:
>There are other buffer-local minor modes that are of similar nature:
>display-line-numbers-mode, display-fill-column-indicator-mode, and
>maybe some others (e.g., what about hl-line-mode?).  So if we are
>going to preserve text-scale-mode across reverting, I think we should
>have a list of modes to preserve, not just exempt this one mode.

Good thinking; thanks, Eli.

So we would have a list variable `revert-buffer-preserved-modes', with a default value that includes:

  `text-scale-mode'
  `display-line-numbers-mode'
  `display-fill-column-indicator-mode'
  `hl-line-mode' (maybe?)
  ...maybe others...

Over time we can adjust the default contents of this list.

One minor implementation detail: some modes may have ancillary information they need to preserve along with the `foo-mode' boolean.  For example, `text-scale-mode' may need `text-scale-mode-amount' to be preserved: toggling `text-scale-mode' directly leaves that variable alone, so that the toggles jump between default text scale and whatever text scale one most recently set explicitly; however, `revert-buffer' causes `text-scale-mode-amount' to be reset back to 0.  I haven't dug into the code yet to see whether this was really a design decision or just an accident of implementation that can be fixed.  If it turns out that some modes need extra information preserved along with the mode variable, then the form of the proposed new list `revert-buffer-preserved-modes' might be a bit more complex than just a list of mode name symbols.  E.g., each element could be either a mode name symbol or a sublist, and in the latter case the first element of the sublist is the mode name symbol and any subsequent elements are mode-specific variables that need to be preserved.

Anyway, that's an implementation detail and can be figured out later.

In general, are you in favor of having a default list of preserved modes for `revert-buffer', with the above modes as the initial candidates for membership in that list?

Best regards,
-Karl