bug#62275: 28.2; Changing major mode changes the current text scaling

[Drew Adams]

Maybe this is intended and documented somewhere - if so, feel free to
close it.  I haven't seen it mentioned, and it doesn't seem right to me.

emacs -Q

Use the mouse (with `C-') or keys to text-scale the buffer, e.g., to
enlarge the text size.

M-x emacs-lisp-mode

The text in the buffer (*scratch*) is resized back to its original size.
Should that happen?  If so, why - is that the right behavior?

In addition, if you then do `M-x lisp-interaction-mode', the text size
doesn't change back to the size you had scaled it to previously.

Text-scaling is per buffer, and the buffer hasn't changed.  Only the
major mode has changed.

I can guess that this is happening because changing the mode kills all
local variables.  But is this the intended behavior, i.e., what we want?
As a user, I find this unexpected (even a bit annoying).  I think users
will (and should) think of text-scaling as per buffer, not something
that's affected by changing the major mode.

I tried doing this, but it didn't fix the behavior.

(put 'text-scale-mode-remapping 'permanent-local t)
(put 'text-scale-mode-lighter 'permanent-local t)
(put 'text-scale-mode-amount 'permanent-local t)

Haven't tried to understand what's really going on - just reporting that
the behavior seems odd to me.

If there's (also) a good use case for the current behavior then maybe we
could have a user option, to choose whether a major-mode change resets
the text scale?

In GNU Emacs 28.2 (build 2, x86_64-w64-mingw32)
 of 2022-09-13 built on AVALON
Windowing system distributor 'Microsoft Corp.', version 10.0.19044
System Description: Microsoft Windows 10 Pro (v10.0.2009.19044.2604)

Configured using:
 'configure --with-modules --without-dbus --with-native-compilation
 --without-compress-install CFLAGS=-O2'

Configured features:
ACL GIF GMP GNUTLS HARFBUZZ JPEG JSON LCMS2 LIBXML2 MODULES NATIVE_COMP
NOTIFY W32NOTIFY PDUMPER PNG RSVG SOUND THREADS TIFF TOOLKIT_SCROLL_BARS
XPM ZLIB

(NATIVE_COMP present but libgccjit not available)