Warnings when showing the selection dialog in the widget-factory from the menubar
#0 _g_log_abort (breakpoint=1) at ../glib/gmessages.c:557
#1 0x00007ffff74e0870 in g_logv (log_domain=0x7ffff746502e "GLib-GObject",
log_level=G_LOG_LEVEL_WARNING, format=0x7ffff7466758 "invalid cast from '%s' to '%s'",
args=0x7fffffffcb90) at ../glib/gmessages.c:1373
#2 0x00007ffff74e0999 in g_log (log_domain=0x7ffff746502e "GLib-GObject",
log_level=G_LOG_LEVEL_WARNING, format=0x7ffff7466758 "invalid cast from '%s' to '%s'")
at ../glib/gmessages.c:1415
#3 0x00007ffff7452804 in g_type_check_instance_cast (type_instance=0x555555ceeb40,
iface_type=93824998244688) at ../gobject/gtype.c:4068
#4 0x00007ffff782020f in gtk_main_do_event (event=0x7fffd8005cc0) at ../gtk/gtkmain.c:1757
#5 0x00007ffff7868edd in surface_event (surface=0x5555574ae5e0, event=0x7fffd8005cc0,
widget=0x555555ceeb40) at ../gtk/gtkpopover.c:766
#6 0x00007ffff7b56a35 in _gdk_marshal_BOOLEAN__POINTERv (closure=0x555557ad5260,
return_value=0x7fffffffd160, instance=0x5555574ae5e0, args=0x7fffffffd210, marshal_data=0x0,
n_params=1, param_types=0x555556617320) at gdk/gdkmarshalers.c:302
#7 0x00007ffff7b9c018 in gdk_surface_event_marshallerv (closure=0x555557ad5260,
return_value=0x7fffffffd160, instance=0x5555574ae5e0, args=0x7fffffffd210, marshal_data=0x0,
n_params=1, param_types=0x555556617320) at ../gdk/gdksurface.c:425
#8 0x00007ffff742a179 in _g_closure_invoke_va (closure=0x555557ad5260, return_value=0x7fffffffd160,
instance=0x5555574ae5e0, args=0x7fffffffd210, n_params=1, param_types=0x555556617320)
at ../gobject/gclosure.c:873
#9 0x00007ffff74476f7 in g_signal_emit_valist (instance=0x5555574ae5e0, signal_id=508, detail=0,
var_args=0x7fffffffd210) at ../gobject/gsignal.c:3403
#10 0x00007ffff744899a in g_signal_emit (instance=0x5555574ae5e0, signal_id=508, detail=0)
at ../gobject/gsignal.c:3550
#11 0x00007ffff7b9b00b in gdk_surface_handle_event (event=0x7fffd8005cc0) at ../gdk/gdksurface.c:2879
#12 0x00007ffff7b76c53 in _gdk_event_emit (event=0x7fffd8005cc0) at ../gdk/gdkevents.c:514
#13 0x00007ffff7bb5dc9 in gdk_event_source_dispatch (base=0x555555b72a00, callback=0x0, data=0x0)
at ../gdk/wayland/gdkeventsource.c:141
#14 0x00007ffff74d6219 in g_main_dispatch (context=0x555555b21880) at ../glib/gmain.c:3309
#15 0x00007ffff74d7108 in g_main_context_dispatch (context=0x555555b21880) at ../glib/gmain.c:3974
#16 0x00007ffff74d72fb in g_main_context_iterate (context=0x555555b21880, block=1, dispatch=1,
self=0x555555b20670) at ../glib/gmain.c:4047
#17 0x00007ffff74d73d3 in g_main_context_iteration (context=0x555555b21880, may_block=1)
at ../glib/gmain.c:4108
#18 0x00007ffff72fca57 in g_application_run (application=0x555555b130e0, argc=1, argv=0x7fffffffd6f8)
at ../gio/gapplication.c:2559
#19 0x000055555555e777 in main (argc=1, argv=0x7fffffffd6f8)
at ../demos/widget-factory/widget-factory.c:2309
Just go to page 2 of the widget-factory and go to the View->Select Background menu item.