bug#44204: Emacs --with-xwidgets complains under Ubuntu 20.04

[Paul Eggert <eggert@cs.ucla.edu>]

On 12/8/21 12:23, Stefan Kangas wrote:
> I guess someone running Ubuntu 20.04 could try reproducing this again
> given that there has been a lot of activity on the xwidget branch
> recently.

I just now checked the current Emacs master (commit 
00d107ebead8d710cbe339d9af717e6eb2f9f2ff) on Ubuntu 21.10 with patches 
current as of today, and things have gotten a bit worse.

Configuring with './configure --with-xwidgets', building and then 
starting with 'emacs -Q', and typing this:

(make-xwidget 'webkit "title" 500 500 '())

followed by C-j, causes the following text to be output to stderr:

-----
Overriding existing handler for signal 10. Set JSC_SIGNAL_FOR_GC if you 
want WebKit to use a different signal

(emacs:66402): Gdk-WARNING **: 13:05:22.877: 
../../../../../gdk/x11/gdkwindow-x11.c:5653 drawable is not a native X11 
window

(emacs:66402): Gdk-WARNING **: 13:05:22.880: 
../../../../../gdk/x11/gdkwindow-x11.c:5653 drawable is not a native X11 
window

(emacs:66402): Gdk-WARNING **: 13:05:22.880: 
../../../../../gdk/x11/gdkwindow-x11.c:5653 drawable is not a native X11 
window
-----

and the following text to be put into Emacs *Messages* buffer:

-----
<xwidget-event> is undefined [4 times]
-----

I am using the default Ubuntu window manager. The command "env | grep 
'^XDG' | sort" outputs:

XDG_CONFIG_DIRS=/etc/xdg/xdg-ubuntu-xorg:/etc/xdg
XDG_CURRENT_DESKTOP=ubuntu:GNOME
XDG_DATA_DIRS=/usr/share/ubuntu-xorg:/usr/local/share/:/usr/share/:/var/lib/snapd/desktop
XDG_MENU_PREFIX=gnome-
XDG_RUNTIME_DIR=/run/user/1000
XDG_SESSION_CLASS=user
XDG_SESSION_DESKTOP=ubuntu-xorg
XDG_SESSION_TYPE=x11