[mutter] stack: Allow promoting transient windows to their parent's layer

[Jonas Ådahl <jadahl src gnome org>]

commit 1c89fce30e0f478f11f90221ddb6b831d7f0f5e8
Author: Florian Müllner <fmuellner gnome org>
Date:   Wed Nov 27 02:00:00 2019 +0100

    stack: Allow promoting transient windows to their parent's layer
    
    When a window that should be stacked above another one is placed in a lower
    layer than the other window, we currently allow promoting it to the higher
    layer when it has a "transient type". We should do the same when the window
    is an actual transient of the other window.
    
    This is particularly relevant for wayland windows, where types play a
    much smaller role: Transient windows like non-modal dialogs (and since
    commit 666bef7a, popup windows as well) currently end up underneath their
    always-on-top parent.
    
    https://gitlab.gnome.org/GNOME/mutter/issues/587

 src/core/stack.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)
---
diff --git a/src/core/stack.c b/src/core/stack.c
index 1279066ff..d7a0c9c67 100644
--- a/src/core/stack.c
+++ b/src/core/stack.c
@@ -859,8 +859,11 @@ static void
 ensure_above (MetaWindow *above,
               MetaWindow *below)
 {
-  if (WINDOW_HAS_TRANSIENT_TYPE(above) &&
-      above->layer < below->layer)
+  gboolean is_transient;
+
+  is_transient = WINDOW_HAS_TRANSIENT_TYPE (above) ||
+                 above->transient_for == below;
+  if (is_transient && above->layer < below->layer)
     {
       meta_topic (META_DEBUG_STACK,
                  "Promoting window %s from layer %u to %u due to contraint\n",