window: Check if we can use CSD before enabling them

The change in 03213b9509fc1df16c66194ea168aed6c15110e9 changed the rules
as to when CSD can be enabled, but it also unconditionally enables CSD
with the implicit assumption that client-side shadows were the real
issue, and that we could work around that by drawing our own borders.
This also means that setting a titlebar for a GtkWindow will enable CSD
unconditionally.

In reality, some window managers (like Matchbox) *only* support
server-side decorations, and will ignore all hints to the contrary, to
the point of drawing decorations at random locations on top of the
window.

Since CSD are enabled unconditionally, the GTK_CSD environment variable
is also not a suitable escape hatch.

In the grand tradition of asking ourselves if we should do something
just because we can, we should split the environment checks from the
checks on what the user requested; by doing that, we can also check
when enabling client-side decorations, and ideally bail out if needed.

https://bugzilla.gnome.org/show_bug.cgi?id=750343

1 files changed, 31 insertions, 19 deletions

diff --git a/gtk/gtkwindow.c b/gtk/gtkwindow.c
index 423c6bd065..9fe882feb7 100644
--- a/gtk/gtkwindow.c
+++ b/gtk/gtkwindow.c
@@ -4056,6 +4056,32 @@ gtk_window_supports_client_shadow (GtkWindow *window)
   return TRUE;
 }
 
+static gboolean
+gtk_window_can_use_csd (GtkWindow *window)
+{
+  const gchar *csd_env;
+
+#ifdef GDK_WINDOWING_BROADWAY
+  if (GDK_IS_BROADWAY_DISPLAY (gtk_widget_get_display (GTK_WIDGET (window))))
+    return TRUE;
+#endif
+
+#ifdef GDK_WINDOWING_WAYLAND
+  if (GDK_IS_WAYLAND_DISPLAY (gtk_widget_get_display (GTK_WIDGET (window))))
+    return TRUE;
+#endif
+
+#ifdef GDK_WINDOWING_MIR
+  if (GDK_IS_MIR_DISPLAY (gtk_widget_get_display (GTK_WIDGET (window))))
+    return TRUE;
+#endif
+
+  csd_env = g_getenv ("GTK_CSD");
+
+  /* If GTK_CSD is unset we default to CSD support */
+  return csd_env == NULL || (strcmp (csd_env, "1") == 0);
+}
+
 static void
 gtk_window_enable_csd (GtkWindow *window)
 {
@@ -4063,6 +4089,10 @@ gtk_window_enable_csd (GtkWindow *window)
   GtkWidget *widget = GTK_WIDGET (window);
   GdkVisual *visual;
 
+  /* If the environment does not support CSD, then there's no point in enabling them */
+  if (!gtk_window_can_use_csd (window))
+    return;
+
   /* We need a visual with alpha for client shadows */
   if (priv->use_client_shadow)
     {
@@ -5839,7 +5869,6 @@ static gboolean
 gtk_window_should_use_csd (GtkWindow *window)
 {
   GtkWindowPrivate *priv = window->priv;
-  const gchar *csd_env;
 
   if (priv->csd_requested)
     return TRUE;
@@ -5850,24 +5879,7 @@ gtk_window_should_use_csd (GtkWindow *window)
   if (priv->type == GTK_WINDOW_POPUP)
     return FALSE;
 
-#ifdef GDK_WINDOWING_BROADWAY
-  if (GDK_IS_BROADWAY_DISPLAY (gtk_widget_get_display (GTK_WIDGET (window))))
-    return TRUE;
-#endif
-
-#ifdef GDK_WINDOWING_WAYLAND
-  if (GDK_IS_WAYLAND_DISPLAY (gtk_widget_get_display (GTK_WIDGET (window))))
-    return TRUE;
-#endif
-
-#ifdef GDK_WINDOWING_MIR
-  if (GDK_IS_MIR_DISPLAY (gtk_widget_get_display (GTK_WIDGET (window))))
-    return TRUE;
-#endif
-
-  csd_env = g_getenv ("GTK_CSD");
-
-  return (g_strcmp0 (csd_env, "1") == 0);
+  return gtk_window_can_use_csd (window);
 }
 
 static void