bug#44466: 27.1; quail input fails at read-only boundary

[Eli Zaretskii <eliz@gnu.org>]

> From: Evgeny Zajcev <lg.zevlg@gmail.com>
> Date: Sun, 8 Nov 2020 09:42:23 +0300
> Cc: dick.r.chiang@gmail.com, 44466@debbugs.gnu.org
> 
> > When an input method is used in a read-only buffer, Emacs barfs
> > because it doesn't allow inserting text into such a buffer.  It
> > doesn't insert the untranslated character, as what your patch did.
> >
> >> Possible we need to check front-stickyness of the char at point along with 'read-only property:
> >>
> >> ..
> >>                   (and (get-char-property (point) 'read-only)
> >>                        (get-char-property (point) 'front-sticky)))
> >
> > Does this solve the problem in this case?
> 
> Yes, because this mimics what is done in
> verify_interval_modification() function from textprop.c.

Any reason why pressing a key on a button should disregard the active
input method?  What if the button needs the user to type the character
which the input method would produce?  AFAIU, the patch we installed a
year ago makes this impossible.

> Possibly the best solution would be to make
> verify_interval_modification() visible from elisp side and use it in
> quail-input-method to check for writability at point.

I think this would be overkill.