bug#19994: 25.0.50; Unicode keyboard input on Windows

[Eli Zaretskii <eliz@gnu.org>]

> Date: Wed, 8 Jul 2015 17:02:59 -0700
> From: Ilya Zakharevich <ilya@math.berkeley.edu>
> Cc: 19994@debbugs.gnu.org
> 
> On Wed, Jul 01, 2015 at 03:07:12AM -0700, Ilya Zakharevich wrote:
> > On Wed, Mar 04, 2015 at 08:01:01PM +0200, Eli Zaretskii wrote:
> 
> > > I suggest, indeed, to clean up the code so we could commit it to the
> > > master branch.  That way, it will get wider testing, and we can fix
> 
> > I had no time to work on the code itself, but
> >   • I fixed the formatting,
> >   • I pumped up the docs,
> >   • I put in the suggested eassert().
> 
> The variant I sent was too primitive — it was not covering a (common?)
> usage case when (with AltGr-layouts) leftCtrl+rightCtrl was behaving
> differently than pressing AltGr:
>    • leftCtrl+rightCtrl would trigger C-M-key;
>    • altGr would enter the character payload.
> 
> This update
> 
>   (0) fixes two formatting-style omissions;
> 
>   (A) adds A LOAD of new comments;
>   (B) treats such important cases (as above) separately;
> 
>   (z) Marks a piece of old code which does not make any sense.
>         (see the last chunk in the relative patch)
> 
> Notes:
> 
>   • In (B), there are some decisions to make.  I encapsulate these
>     decisions into two strings.  For best result, these strings should
>     be user-customizable.  However, currently they are just put into
>     C #defines.
> 
>     When I sit on this more, and if these customizations turn out to
>     be useful, one can make them into Lisp variables.
> 
>   • There is a bug in the (old) Emacs code which prevents some cases
>     treated in (B) from being really useful.  I did not fix it yet.
> 
>     To see the bug:
>       ∘ switch to layout with AltGr;
>       ∘ assume that AltGr-s produces ß (as with US International);
>       ∘ pressing AltGr-rightControl-s produces Meta-ß;
>       ∘ pressing rightControl-AltGr-s produces C-M-s.
>     (I do not think this effect is intentional.)
> 
>   • And, BTW, is it documented anywhere that
>     leftControl-rightControl-key produces C-M-key?
> 
> I include two patches:
>   □   absolute (ignore the previous patches)
>   □   relative (with whitespace ignored) — for reading.

Thanks.  I committed this in your name, with a few minor stylistic
changes, and also fixed a few typos in the comments.  Sorry for a long
delay in doing that.

I also added a new variable, w32-use-fallback-wm-chars-method, which,
when non-nil, makes Emacs use the old code from before your changes.
This is meant to be a handy debugging aid, in case we discover some
issues with the new code.

Do you think there are any user-visible effects of your changes that
are worthy of mentioning in NEWS?  If so, please propose the text for
NEWS.

I leave it up to you to decide whether this bug should be closed, or
if there's something else to be done about it.

Thanks again for working on this.