bug#64347: 30.0.50; Some customize faces shown as edited with -Q

[Eli Zaretskii <eliz@gnu.org>]

> Date: Sun, 9 Jul 2023 08:44:23 -0300
> Cc: 64347@debbugs.gnu.org, stephen.berman@gmx.net
> From: Mauro Aranda <maurooaranda@gmail.com>
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
>  > I guess I'm missing something here: why do we need those pre-filter
>  > and post-filter conversions?  The C code understands both forms of
>  > :line-width, so there should be no need for Lisp to do any
>  > conversions, right?  So why do we do that? why not simply leave the
>  > spec as it was originally?
> 
> Custom needs the pre-filter in order to present a Custom buffer to edit
> the face.
> Let's say there's a face:
> (defface foo
>    '((t (:box (:line-width 1 :color "black"))))
>    "...")
> 
> And let's say a user wants to customize it via Custom.
> M-x customize-face RET foo
> should show the user a buffer with all the capabilities to edit it.
> Because we have an integer for the :line-width property, the user will
> be presented with an integer Widget to edit the value, without giving
> the user the opportunity to edit the HWIDTH and VWIDTH separately.
> 
> So the pre-filter takes the (:line-width 1), and converts it into
> (:line-width (1 . 1)), and now the Custom buffer will have
> a cons Widget.  If we didn't do that conversion, that would be a Bug
> report, I'm sure.

OK, but why does it have to do that on the original value?  It could
do that on a copy that serves for the display and editing, in which
case the original value could be left intact if the user didn't change
it or did change, but didn't click Apply.  (If the user does modify
the original value, then any conversions are okay, since the variable
is really "edited".)