wayland: prefer subsurface when possible

Quite a few applications use GTK_WINDOW_POPUP to create various
temporary windows and place then on screen. That works fine on X11 but
on Wayland there is no global coordinate system for regular surfaces.

If the application is using a gdk temp window and set a parent with
gtk_window_transient_for(), the gdk wayland backend has all it needs to
create a subsurface that can be placed at will by the application.

Bugzilla: https://bugzilla.gnome.org/show_bug.cgi?id=759738

gdk/wayland/gdkwindow-wayland.c

@@ -1227,36 +1227,27 @@ should_be_mapped (GdkWindow *window)
   return TRUE;
 }
 
-static gboolean
-should_map_as_subsurface (GdkWindow *window)
-{
-  GdkWindowImplWayland *impl = GDK_WINDOW_IMPL_WAYLAND (window->impl);
-
-  if (GDK_WINDOW_TYPE (window) == GDK_WINDOW_SUBSURFACE)
-    return TRUE;
-
-  switch (impl->hint)
-    {
-    case GDK_WINDOW_TYPE_HINT_TOOLTIP:
-      return TRUE;
-
-    case GDK_WINDOW_TYPE_HINT_UTILITY:
-      if (GDK_WINDOW_TYPE (window) == GDK_WINDOW_TEMP)
-        return TRUE;
-      break;
-
-    default:
-      break;
-    }
-
-  return FALSE;
-}
-
 static gboolean
 should_map_as_popup (GdkWindow *window)
 {
   GdkWindowImplWayland *impl = GDK_WINDOW_IMPL_WAYLAND (window->impl);
 
+  /* Ideally, popup would be temp windows with a parent and grab */
+  if (GDK_WINDOW_TYPE (window) == GDK_WINDOW_TEMP)
+    {
+      /* If a temp window has a parent and a grab, we can use a popup */
+      if (impl->transient_for)
+        {
+          if (impl->grab_input_seat)
+            return TRUE;
+        }
+      else
+        g_warning ("Window %p is a temporary window without parent, "
+                   "application will not be able to position it on screen.",
+                   window);
+    }
+
+  /* Yet we need to keep the window type hint tests for compatibility */
   switch (impl->hint)
     {
     case GDK_WINDOW_TYPE_HINT_POPUP_MENU:
@@ -1276,6 +1267,38 @@ should_map_as_popup (GdkWindow *window)
   return FALSE;
 }
 
+static gboolean
+should_map_as_subsurface (GdkWindow *window)
+{
+  GdkWindowImplWayland *impl = GDK_WINDOW_IMPL_WAYLAND (window->impl);
+
+  if (GDK_WINDOW_TYPE (window) == GDK_WINDOW_SUBSURFACE)
+    return TRUE;
+
+  if (GDK_WINDOW_TYPE (window) != GDK_WINDOW_TEMP)
+    return FALSE;
+
+  /* if we want a popup, we do not want a subsurface */
+  if (should_map_as_popup (window))
+    return FALSE;
+
+  if (impl->transient_for)
+    {
+      GdkWindowImplWayland *impl_parent;
+
+      impl_parent = GDK_WINDOW_IMPL_WAYLAND (impl->transient_for->impl);
+      /* subsurface require that the parent is mapped */
+      if (impl_parent->mapped)
+        return TRUE;
+      else
+        g_warning ("Couldn't map window %p as susburface because its parent is not mapped.",
+                   window);
+
+    }
+
+  return FALSE;
+}
+
 /* Get the window that can be used as a parent for a popup, i.e. a xdg_surface
  * or xdg_popup. If the window is not, traverse up the transiency parents until
  * we find one.
@@ -1313,7 +1336,7 @@ gdk_wayland_window_map (GdkWindow *window)
       if (impl->transient_for)
         gdk_wayland_window_create_subsurface (window);
       else
-        g_warning ("Couldn't map as window %p as susburface yet because it doesn't have a parent",
+        g_warning ("Couldn't map window %p as susburface yet because it doesn't have a parent",
                    window);
     }
   else if (should_map_as_popup (window))

gtk/gtkwindow.c

@@ -3229,6 +3229,12 @@ gtk_window_unset_transient_for  (GtkWindow *window)
  *
  * Passing %NULL for @parent unsets the current transient window.
  *
+ * On Wayland, this function can also be used to attach a new
+ * #GTK_WINDOW_POPUP to a #GTK_WINDOW_TOPLEVEL parent already mapped
+ * on screen so that the #GTK_WINDOW_POPUP will be created as a
+ * subsurface-based window #GDK_WINDOW_SUBSURFACE which can be
+ * positioned at will relatively to the #GTK_WINDOW_TOPLEVEL surface.
+ *
  * On Windows, this function puts the child window on top of the parent,
  * much as the window manager would have done on X.
  */

tests/Makefile.am

@@ -170,6 +170,7 @@ noinst_PROGRAMS =  $(TEST_PROGS)	\
 	gdkgears			\
 	listmodel			\
 	foreigndrawing			\
+	testpopup			\
 	$(NULL)
 
 if USE_X11

tests/testpopup.c

@@ -0,0 +1,68 @@
+#include <gtk/gtk.h>
+
+static gboolean
+draw_popup (GtkWidget *widget,
+            cairo_t   *cr,
+            gpointer   data)
+{
+  cairo_set_source_rgb (cr, 1, 0, 0);
+  cairo_paint (cr);
+
+  return FALSE;
+}
+
+static gboolean
+place_popup (GtkWidget *parent,
+             GdkEvent  *event,
+             GtkWidget *popup)
+{
+  GdkEventMotion *ev_motion = (GdkEventMotion *) event;
+  gint width, height;
+
+  gtk_window_get_size (GTK_WINDOW (popup), &width, &height);
+  gtk_window_move (GTK_WINDOW (popup),
+                   (int) ev_motion->x_root - width / 2,
+                   (int) ev_motion->y_root - height / 2);
+
+  return FALSE;
+}
+
+static gboolean
+on_map_event (GtkWidget *parent,
+              GdkEvent  *event,
+              gpointer   data)
+{
+  GtkWidget *popup;
+
+  popup = gtk_window_new (GTK_WINDOW_POPUP);
+
+  gtk_widget_set_size_request (GTK_WIDGET (popup), 20, 20);
+  gtk_widget_set_app_paintable (GTK_WIDGET (popup), TRUE);
+  gtk_window_set_transient_for (GTK_WINDOW (popup), GTK_WINDOW (parent));
+  g_signal_connect (popup, "draw", G_CALLBACK (draw_popup), NULL);
+  g_signal_connect (parent, "motion-notify-event", G_CALLBACK (place_popup), popup);
+
+  gtk_widget_show (popup);
+
+  return FALSE;
+}
+
+int
+main (int argc, char *argv[])
+{
+  GtkWidget *window;
+
+  gtk_init (&argc, &argv);
+
+  window = gtk_window_new (GTK_WINDOW_TOPLEVEL);
+
+  gtk_widget_set_events (window, GDK_POINTER_MOTION_MASK);
+  g_signal_connect (window, "destroy", gtk_main_quit, NULL);
+  g_signal_connect (window, "map-event", G_CALLBACK (on_map_event), NULL);
+
+  gtk_widget_show (window);
+
+  gtk_main ();
+
+  return 0;
+}