From commit 0611cb4823c the following was changed:

@@ -877,11 +850,16 @@ gdk_screen_get_monitor_workarea (GdkScreen    *screen,
                                  gint          monitor_num,
                                  GdkRectangle *dest)
 {
+  GdkMonitor *monitor;
+
   g_return_if_fail (GDK_IS_SCREEN (screen));
-  g_return_if_fail (monitor_num >= 0);
-  g_return_if_fail (monitor_num < gdk_screen_get_n_monitors (screen));
 
-  GDK_SCREEN_GET_CLASS (screen)->get_monitor_workarea (screen, monitor_num, dest);
+  monitor = get_monitor (screen, monitor_num);
+
+  g_return_if_fail (monitor != NULL);
+
+  /* FIXME */
+  gdk_monitor_get_geometry (monitor, dest);
 }

This means that gdk_screen_get_monitor_workarea no longer gets the work area.