frames: Load libadwaita on GNOME

Or, optionally, when the MUTTER_FRAMES_PLATFORM_LIBRARY env var is set
to "adwaita". For debugging purposes, it is possible to disable loading
any platform library with MUTTER_FRAMES_PLATFORM_LIBRARY set to "none".

Add the CSS class "ssd-frame" to the MetaFrame (i.e. GtkWindow)
instances
as that lets libadwaita pick the right style, and won't do anything for
non-Adwaita styles.

This patch is specially careful not to link against libadwaita.

Closes https://gitlab.gnome.org/GNOME/mutter/-/issues/2830

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3981>

src/frames/main.c

@@ -23,8 +23,49 @@
 
 #include <gdk/x11/gdkx.h>
 #include <glib-unix.h>
+#include <gmodule.h>
 #include <X11/extensions/Xfixes.h>
 
+typedef void (* InitFunc) (void);
+
+static gboolean
+should_load_libadwaita (void)
+{
+  g_auto(GStrv) desktops = NULL;
+  const char *current_desktop;
+  const char *platform_library;
+
+  platform_library = g_getenv ("MUTTER_FRAMES_PLATFORM_LIBRARY");
+
+  if (g_strcmp0 (platform_library, "none") == 0)
+    return FALSE;
+
+  if (g_strcmp0 (platform_library, "adwaita") == 0)
+    return TRUE;
+
+  current_desktop = g_getenv ("XDG_CURRENT_DESKTOP");
+  if (current_desktop != NULL)
+    desktops = g_strsplit (current_desktop, ":", -1);
+
+  return desktops && g_strv_contains ((const char * const *) desktops, "GNOME");
+}
+
+static void
+load_libadwaita (GdkDisplay *display)
+{
+  GModule *libadwaita;
+  InitFunc adw_init;
+
+  libadwaita = g_module_open ("libadwaita-1.so.0", G_MODULE_BIND_LAZY);
+  if (!libadwaita)
+    return;
+
+  if (!g_module_symbol (libadwaita, "adw_init", (gpointer *) &adw_init))
+    return;
+
+  adw_init ();
+}
+
 static gboolean
 on_sigterm (gpointer user_data)
 {
@@ -57,6 +98,9 @@ main (int   argc,
 
   display = gdk_display_get_default ();
 
+  if (should_load_libadwaita ())
+    load_libadwaita (display);
+
   xdisplay = gdk_x11_display_get_xdisplay (display);
   XFixesSetClientDisconnectMode (xdisplay,
                                  XFixesClientDisconnectFlagTerminate);

src/frames/meson.build

@@ -9,6 +9,7 @@ x11_frames_sources = [
 x11_frames = executable('mutter-x11-frames',
   sources: x11_frames_sources,
   dependencies: [
+    gmodule_no_export_dep,
     gsettings_desktop_schemas_dep,
     gtk4_dep,
     x11_dep,

src/frames/meta-frame.c

@@ -527,6 +527,8 @@ meta_frame_class_init (MetaFrameClass *klass)
 static void
 meta_frame_init (MetaFrame *frame)
 {
+  gtk_widget_add_css_class (GTK_WIDGET (frame), "ssd-frame");
+
   g_signal_connect (frame, "close-request",
                     G_CALLBACK (on_frame_close_request), NULL);
 }