[mutter] wayland/subsurface: Only show subsurface if parent is mapped

[Jonas Ådahl <jadahl src gnome org>]

commit ae0d48b6fd3aed3e75a25ef1f749c4620e2e3e57
Author: Robert Mader <robert mader posteo de>
Date:   Wed Oct 14 18:50:05 2020 +0200

    wayland/subsurface: Only show subsurface if parent is mapped
    
    The spec states:
    ```
    A sub-surface becomes mapped, when a non-NULL wl_buffer is applied
    and the parent surface is mapped. The order of which one happens
    first is irrelevant. A sub-surface is hidden if the parent becomes hidden,
    or if a NULL wl_buffer is applied. These rules apply recursively
    through the tree of surfaces.
    ```
    
    In the past we relied on Clutter actor behaviour to realize the recursive
    part - which then broke in
    https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/664
    when we changed the actor hierachy in regards to subsurfaces.
    
    Explicitly encode the desired behaviour in `MetaWaylandSubsurface`, fixing
    the issue and making it future proof.
    
    Closes https://gitlab.gnome.org/GNOME/mutter/-/issues/1384

 src/wayland/meta-wayland-subsurface.c | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)
---
diff --git a/src/wayland/meta-wayland-subsurface.c b/src/wayland/meta-wayland-subsurface.c
index bf14e0e3c9..b4c503d201 100644
--- a/src/wayland/meta-wayland-subsurface.c
+++ b/src/wayland/meta-wayland-subsurface.c
@@ -67,6 +67,17 @@ transform_subsurface_position (MetaWaylandSurface *surface,
   while (surface);
 }
 
+static gboolean
+should_show (MetaWaylandSurface *surface)
+{
+  if (!surface->buffer_ref->buffer)
+    return FALSE;
+  else if (surface->sub.parent)
+    return should_show (surface->sub.parent);
+  else
+    return TRUE;
+}
+
 static void
 sync_actor_subsurface_state (MetaWaylandSurface *surface)
 {
@@ -87,7 +98,7 @@ sync_actor_subsurface_state (MetaWaylandSurface *surface)
   clutter_actor_set_position (actor, x, y);
   clutter_actor_set_reactive (actor, TRUE);
 
-  if (surface->buffer_ref->buffer)
+  if (should_show (surface))
     clutter_actor_show (actor);
   else
     clutter_actor_hide (actor);