bug#67430: 29.1; <Multi_key> is undefined

[Francesco Potortì]

>> From: Po Lu <luangruo@yahoo.com>
>> Cc: pot@gnu.org,  67430@debbugs.gnu.org
>> Date: Sat, 25 Nov 2023 19:10:15 +0800
>> 
>> Eli Zaretskii <eliz@gnu.org> writes:
>> 
>> > What other roles does this key play?  And how frequent is each role?
>> 
>> I don't know.  There is generally no other use for it under X, except
>> perhaps as a modifier key, which if true Emacs won't register key
>> presses at all.
>> 
>> > Also, if we bind this key by default as Francesco suggests, what
>> > adverse results could this cause on the systems where this binding is
>> > wrong?
>> 
>> Nothing beyond the obvious, to wit: Emacs will react to pressing the
>> Multi_key as though it were bound to iso-transl-ctl-x-8-map.
>
>Then maybe we should behave by default as Francesco suggested?  users
>which don't like the results could always unbind/rebind the key.
>WDYT?

In mormal usage, Emacs never even sees <Multi_key>.  I suppose that Emacs, like 
usual X applications, relies on X to interpret it as a modifier key, and X uses 
its composition rules.  But Xpra apparently is another piece of cake (at least 
the version I am using) and it passes the key as-is.  I think that this is 
unusual behaviour.

If Emacs happens to see <Multi_key>, maybe in principle it should ask X to 
interpret the subsequent characters using X's composition rules.  I don't know 
if that's possible in a clean way, I suppose that would not be obvious to do.  
Lacking such functionality, I think the only reasonable thing to do is to 
interpretet is as C-x 8, which has composition rules mostly compatible with 
those of X.  So in most cases this will be transparent to the user.  I don't 
see how throwing an error instead can be better.  And yes, in principle users 
can rebind it.

An alternative would be to bind <Multi_key> to a command which is disabled by 
default (like for example narrow-to-page), and when enabled does the same as 
C-x 8.  But I think that binding it by default is simpler and has so rare and 
technical drawbacks that people caring it about are aware of the issue and can 
solve it.