stacking_list = g_list_append(stacking_list, client);
g_assert(!g_hash_table_lookup(client_map, &client->window));
g_hash_table_insert(client_map, &client->window, client);
stacking_list = g_list_append(stacking_list, client);
g_assert(!g_hash_table_lookup(client_map, &client->window));
g_hash_table_insert(client_map, &client->window, client);