[mutter] wayland/surface: The same sub-surface is not a valid sibling

[Marge Bot <marge-bot src gnome org>]

commit 709d06e89e858cf907e17b5104b4cfa74486dd77
Author: Michel Dänzer <mdaenzer redhat com>
Date:   Fri Jun 17 16:33:17 2022 +0200

    wayland/surface: The same sub-surface is not a valid sibling
    
    The Wayland protocol documentation at
    https://wayland.app/protocols/wayland#wl_subsurface:request:place_above
    says:
    
     The reference surface must be one of the sibling surfaces, or the
     parent surface. Using any other surface, including this sub-surface,
     will cause a protocol error.
    
    v2:
    * Leave it as a single line. (Georges Basile Stavracas Neto)
    
    Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2525>

 src/wayland/meta-wayland-subsurface.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/src/wayland/meta-wayland-subsurface.c b/src/wayland/meta-wayland-subsurface.c
index 4055e68820..3e088b04fb 100644
--- a/src/wayland/meta-wayland-subsurface.c
+++ b/src/wayland/meta-wayland-subsurface.c
@@ -102,7 +102,7 @@ static gboolean
 is_sibling (MetaWaylandSurface *surface,
             MetaWaylandSurface *sibling)
 {
-  return surface->sub.parent == sibling->sub.parent;
+  return surface != sibling && surface->sub.parent == sibling->sub.parent;
 }
 
 static gboolean