bug#33749: 26.1; input-decode-map to empty vector should preserve echo area

[Eli Zaretskii]

> From: Yuri Khan <yurivkhan@gmail.com>
> Date: Sat, 15 Dec 2018 15:07:03 +0700
> Cc: 33749@debbugs.gnu.org
> 
> Okay. So, amended proposed course:
> 
> * Add a variable such as ‘input-decode-preserve-echo’, boolean,
> default nil, that controls the following behavior changes.
> 
> [any better ideas on the naming?]
> 
> * If the above variable is true
> > > and ‘read_key_sequence’ detects that the whole key sequence has been
> > > rewritten to an empty sequence, return -1.
> > >
> > > * In ‘command_loop_1’, save the echo area contents before calling
> > > ‘read_key_sequence’.
> Afterwards, if ‘input-decode-preserve-echo’ is true and the return
> value is -1, restore the echo area contents.
> 
> * In the future ‘terminal-init-xterm-kitty’ function, set
> ‘input-decode-preserve-echo’ to true when full keyboard mode is
> requested.

SGTM, but let's hear what others think.

> An alternative way could be to add hooks at start of
> ‘read_key_sequence’ and at each each sequence rewrite, but IMO that
> would be excessive.

I think I agree.