bug#81666: 31.1; browse-url fails in emacsclient -nw

> From: Abdulnafé Toulaïmat
>  <[email protected]>
> Cc: Po Lu <[email protected]>,  Stefan Monnier <[email protected]>,
>   martin rudalics <[email protected]>,  Yuuki Harano <[email protected]>,
>   [email protected]
> Date: Fri, 21 Aug 2026 08:55:36 +0200
> 
> Eli Zaretskii <[email protected]> writes:
> 
> > This error is specific to a PGTK build, and comes from browse-url here:
> >
> >     ;; When connected to various displays, be careful to use the display of
> >     ;; the currently selected frame, rather than the original start display,
> >     ;; which may not even exist any more.
> >     (let ((dpy (frame-parameter nil 'display))
> >           classname)
> >       (if (stringp dpy)
> >         (cond
> >          ((featurep 'pgtk)
> >           (setq classname (pgtk-backend-display-class))  <<<<<<<<<<<<<<<
> >           (if (equal classname "GdkWaylandDisplay")
> >               (progn
> >                 ;; The `display' frame parameter is probably wrong.
> >                 ;; See bug#53969 for some context.
> >                 ;; (setenv "WAYLAND_DISPLAY" dpy)
> >                 )
> >             (setenv "DISPLAY" dpy)))
> >
> > All this is conditioned on "(frame-parameter nil 'display)" returning
> > a string, which AFAIU is not expected in the -nw case.  So the first
> > question is: what does "(frame-parameter nil 'display)" yield in your
> > case, in the scenario you describe, just before you invoke browse-url
> > from the client frame?
> >
> > (FWIW, I tried to reproduce this in a PGTK build I have here, but
> > couldn't: I see no error in this case.  And frame-parameter returns
> > nil in my case, as expected.)
> 
> Hello, thanks for looking into this.
> 
> In the emacsclient TTY frame, I get ":0". This does indeed appear to be
> the main culprit, because in an *emacs* TTY frame, I get nil, as you
> describe.

So the next question is: how come that value of ":0" appears in a TTY
frame?  Can anyone explain that?

Regardless, we could apply a band-aid, whereby we condition this
fragment on the frame being a GUI frame, in addition to testing the
value of the parameter to be a string.