frames: Add default-decoration CSS class

Add this CSS class both to the header bar itself, since it is what
actually contains the window controls, and to MetaFrameHeader too,
since it's what's directly attached to the window.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2740>

2 files changed, 3 insertions, 0 deletions

diff --git a/src/frames/meta-frame-header.c b/src/frames/meta-frame-header.c
index 5f2cda1c5..e5b0b5b9f 100644
--- a/src/frames/meta-frame-header.c
+++ b/src/frames/meta-frame-header.c
@@ -115,6 +115,8 @@ meta_frame_header_init (MetaFrameHeader *content)
   GtkWidget *header_bar;
 
   header_bar = gtk_header_bar_new ();
+  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);
 }
 
diff --git a/src/frames/meta-frame.c b/src/frames/meta-frame.c
index 7b55e74e5..66f2109c3 100644
--- a/src/frames/meta-frame.c
+++ b/src/frames/meta-frame.c
@@ -228,6 +228,7 @@ 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);