bug#69941: 30.0.50; Faulty fontification of radio button widgets

[Stephen Berman via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

On Thu, 09 May 2024 10:15:29 -0400 Stefan Monnier <monnier@iro.umontreal.ca> wrote:

>> I investigated the history of this code, and while the value t for the
>> marker insertion type was used in the initial commit, it was changed to
>> nil in commit e0f956935, with the message "Insert new text at the :from
>> marker _after_ the marker, not before it."  But 18 days later it was
>> changed back to t in commit 3bff434b8, that also added "Document need to
>> put some text before the %v escape in :format string" of editable-field
>> widgets.  (I looked at the bug-gnu-emacs and emacs-devel mailing list
>> archives but found nothing relevant at the time just prior to these
>> commits.)
>
> I'm really not familiar with the widget code, but looking around that
> code I see that we have
>
>        (set-marker-insertion-type BLAfromBLA t)
>        (set-marker-insertion-type BLAtoBLA nil)
>
> at various places, and I think that makes a lot of sense when you
> consider that we don't want text inserted right before or right after
> the widget to suddenly become part of the widget.

Yes, Mauro Aranda's response to my OP also made this point, though not
so explicitly, so I think it sort of went by me, but after reading your
comment, I did some further testing and, indeed, my latest patch (I've
only tested that but the other versions I've posted should not differ in
this respect) does result in text inserted directly before a checklist
or radio-button-choice widget being after the widget's :from marker,
which makes the text get covered by the widget-inactive overlay, which
it should not be.

So it seems that either my patch is not the right fix for the
fontification problem, or it has to be accompanied by further
adjustments.  I've briefly looked at the latter approach, thinking that
using insert-before-markers might help, but my attempts have failed so
far.

> But OTOH while "printing" the widget itself, we'd want the exact
> opposite (i.e. nil for from and t for to).
>
> Could it be that part of the problem is that the insertion of
> a radio-button widget into a radio-button-choice widget is done "too
> late", i.e. after the radio-button-choice widget has been printed?

To avoid this I suspect that the radio-button-choice widget would have
to be redesigned, and also the checklist widget, which has a similar
issue.  Although, it seems that a viable alternative to using checklist
widgets may be simply to use checkbox widgets, as e.g. recentf-edit-list
does.  But radio buttons have to be grouped, since only one per group
can be selected.  How to do this without using an enclosing widget like
radio-button-choice, which by definition has it's own :from and :to
properties, I don't know.

Steve Berman