frames: Add CSS class on init

Adding the 'default-decoration' CSS class to MetaFrameHeader after
it is set as the headerbar makes it not account for the minimum size
correctly sometimes. This is a bit racy though - if the window opens
very quickly, it works as expected.

Adding the CSS class before the widget is used guarantees it'll
always report the correct size though, so do that.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2753>

src/frames/meta-frame-header.c

@@ -118,6 +118,8 @@ meta_frame_header_init (MetaFrameHeader *content)
   gtk_widget_add_css_class (header_bar, "titlebar");
   gtk_widget_add_css_class (header_bar, "default-decoration");
   gtk_widget_insert_before (header_bar, GTK_WIDGET (content), NULL);
+
+  gtk_widget_add_css_class (GTK_WIDGET (content), "default-decoration");
 }
 
 GtkWidget *

src/frames/meta-frame.c

@@ -228,7 +228,6 @@ meta_frame_new (Window window)
   header = meta_frame_header_new ();
 
   gtk_window_set_titlebar (GTK_WINDOW (frame), header);
-  gtk_widget_add_css_class (frame, "default-decoration");
 
   content = meta_frame_content_new (window);
   gtk_window_set_child (GTK_WINDOW (frame), content);