Re: Set X primary selection with Emacs in xterm

[Eli Zaretskii <eliz@gnu.org>]

> From: Duncan Findlay <duncf@google.com>
> Date: Fri, 10 Jun 2022 11:10:50 -0700
> Cc: emacs-devel@gnu.org
> 
> On Thu, Jun 2, 2022 at 11:56 PM Eli Zaretskii <eliz@gnu.org> wrote:
> 
> > Thanks.  I think we should solve this differently.  I don't think it's
> > a good idea to call arbitrary Lisp from input-processing loop in
> > keyboard.c, anymore than we already do (which is already too much,
> > IMNSHO), especially if we envision advices for that code.
> >
> > We should instead modify the condition in command_loop_1 to support
> > terminals that can set GUI selections.  terminal-parameter is a
> > primitive written in C, so command_loop_1 could call it directly (it
> > should also pay attention to the defcustom described below).
> 
> I considered this, but given that we're making the same decision in
> lisp/simple.el (deactivate-mark) using display-selections-p, the
> benefits of sharing an implementation seemed compelling.

That's not what bothers me, as I explain above.  I don't want us to
call more Lisp from the loop that processes keyboard input, if that
can be avoided.  And in this case, it can be easily avoided.

> I see your point about wanting to minimize lisp in the command loop.
> Can we just port display-selections-p to C and use it from both
> places, or will that break things?

I see no need for doing that.  display-selections-p is okay in Lisp,
as it is called from Lisp programs.
> > I think TRT here is to provide a defcustom, so that users could
> > disable this feature if it causes more trouble than it's worth.  With
> > time, perhaps we will collect enough user experience to come up with
> > the default value that makes the most sense on most supported systems;
> > for now setting the X selection could just be disabled by default.
> 
> My initial resistance to a defcustom was because this feature already
> requires xterm support for setSelection, which is already somewhat
> rare, and it's already controlled by `select-active-regions'. Without
> a new defcustom, it can be turned off with:
> (add-hook 'terminal-init-xterm-hook (lambda () (setq
> select-active-regions nil)))
> 
> But I accept the feedback that this is not discoverable, and people
> want to avoid surprises, so having a defcustom that's off by default
> makes sense. I'll upload a new version of the patch to the tracker
> shortly.

Thanks.

> > > +     ((and (memq frame-type '(t))
> > > +           (eq (terminal-parameter nil 'xterm--set-selection) t))
> > > +      t)
> >
> > This is unnecessarily strict: there should be no need to test
> > frame-type, since any frame type could arrange for this parameter when
> > it supports selections.
> 
> In practice, are there other frame types? Is it reasonable to set
> terminal-parameter for other frame types?

Maybe not today, but I'd like this code to be more future-proof.
There's no need to test for more things that are absolutely necessary,
and testing for the xterm--set-selection parameter is enough in this
case, isn't it?