dialog: Use default decoration for non-custom headerbars

There are two ways GTK can add a headerbar to a dialog:

 - the dialog is constructed with the :use-header-bar property

 - all windows should use client-side decorations

In the first case, the headerbar is added by GtkDialog with no
dedicated style class, and in the latter by GtkWindow with the
"default-decoration" style.

As a result, dialogs with plain titlebars can end up with clearly
distinct and inconsistent styles.

To address this, allow headerbars to track whether they should use
the "default-decoration" style and enable it for dialogs.

https://gitlab.gnome.org/GNOME/gtk/merge_requests/836

1 files changed, 31 insertions, 0 deletions

diff --git a/gtk/gtkheaderbar.c b/gtk/gtkheaderbar.c
index dcdcb4f4fd..aca524df10 100644
--- a/gtk/gtkheaderbar.c
+++ b/gtk/gtkheaderbar.c
@@ -85,6 +85,7 @@ struct _GtkHeaderBarPrivate
   gboolean show_title_buttons;
   gchar *decoration_layout;
   gboolean decoration_layout_set;
+  gboolean track_default_decoration;
 
   GtkWidget *titlebar_start_box;
   GtkWidget *titlebar_end_box;
@@ -499,6 +500,30 @@ _gtk_header_bar_shows_app_menu (GtkHeaderBar *bar)
   return priv->show_title_buttons && priv->shows_app_menu;
 }
 
+static void
+update_default_decoration (GtkHeaderBar *bar)
+{
+  GtkHeaderBarPrivate *priv = gtk_header_bar_get_instance_private (bar);
+  GtkStyleContext *context;
+
+  context = gtk_widget_get_style_context (GTK_WIDGET (bar));
+
+  if (priv->children != NULL || priv->custom_title != NULL)
+    gtk_style_context_remove_class (context, "default-decoration");
+  else
+    gtk_style_context_add_class (context, "default-decoration");
+}
+
+void
+_gtk_header_bar_track_default_decoration (GtkHeaderBar *bar)
+{
+  GtkHeaderBarPrivate *priv = gtk_header_bar_get_instance_private (bar);
+
+  priv->track_default_decoration = TRUE;
+
+  update_default_decoration (bar);
+}
+
 /* As an intended side effect, this function allows @child
  * to be the title/label box */
 static void
@@ -1543,6 +1568,9 @@ gtk_header_bar_pack (GtkHeaderBar *bar,
   g_signal_connect (widget, "notify::visible", G_CALLBACK (notify_child_cb), bar);
 
   _gtk_header_bar_update_separator_visibility (bar);
+
+  if (priv->track_default_decoration)
+    update_default_decoration (bar);
 }
 
 static void
@@ -1595,6 +1623,9 @@ gtk_header_bar_remove (GtkContainer *container,
       g_free (child);
       gtk_widget_queue_resize (GTK_WIDGET (container));
       _gtk_header_bar_update_separator_visibility (bar);
+
+      if (priv->track_default_decoration)
+        update_default_decoration (bar);
     }
 }