[mutter] tests/test-client: Remove shadow from X11 test client CSS style

[Georges Basile Stavracas Neto <gbsneto src gnome org>]

commit 48de81b63ec1ea6ce0c0aaacf8b47e756caeae2a
Author: Jonas Ådahl <jadahl gmail com>
Date:   Thu Apr 2 16:05:54 2020 +0200

    tests/test-client: Remove shadow from X11 test client CSS style
    
    Gtk is quite buggy and "fluid" in how it handles the shadow margins for
    windows under X11. The "size" of the window fluctuate between including and
    excluding a shadow margin in a way that causes issues, as there are no
    atomic update of any state going on.
    
    In order to avoid running into those particular issues now, lets get rid
    of shadows so the margins are always zero, when the client is using the
    X11 backend.
    
    https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1171

 src/tests/test-client.c | 26 ++++++++++++++++++++++++++
 1 file changed, 26 insertions(+)
---
diff --git a/src/tests/test-client.c b/src/tests/test-client.c
index 2c19d9a36..8b37a4225 100644
--- a/src/tests/test-client.c
+++ b/src/tests/test-client.c
@@ -813,6 +813,32 @@ main(int argc, char **argv)
 
   gtk_init (NULL, NULL);
 
+  if (!wayland)
+    {
+      GdkScreen *screen;
+      GtkCssProvider *provider;
+
+      screen = gdk_screen_get_default ();
+      provider = gtk_css_provider_new ();
+      static const char *no_decoration_css =
+        "decoration {"
+        "  border-radius: 0 0 0 0;"
+        "  border-width: 0;"
+        "  box-shadow: 0 0 0 0 rgba(0, 0, 0, 0), 0 0 0 0 rgba(0, 0, 0, 0);"
+        "  margin: 0px;"
+        "}";
+      if (!gtk_css_provider_load_from_data (provider,
+                                            no_decoration_css,
+                                            strlen (no_decoration_css),
+                                            &error))
+        {
+          g_printerr ("%s", error->message);
+          return 1;
+        }
+      gtk_style_context_add_provider_for_screen (screen, GTK_STYLE_PROVIDER (provider),
+                                                 GTK_STYLE_PROVIDER_PRIORITY_APPLICATION);
+    }
+
   windows = g_hash_table_new_full (g_str_hash, g_str_equal,
                                    g_free, NULL);
   event_source_quark = g_quark_from_static_string ("event-source");