Remove duplicate function declaration and add assertion

include/sway/output.h

@@ -97,6 +97,4 @@ void output_drag_icons_for_each_surface(struct wl_list *drag_icons,
 	struct sway_output *output, struct root_geometry *geo,
 	wlr_surface_iterator_func_t iterator, void *user_data);
 
-struct sway_container *output_get_active_workspace(struct sway_output *output);
-
 #endif

sway/tree/container.c

@@ -1052,6 +1052,10 @@ static struct sway_container *container_floating_find_output(
 
 void container_floating_move_to(struct sway_container *con,
 		double lx, double ly) {
+	if (!sway_assert(container_is_floating(con),
+			"Expected a floating container")) {
+		return;
+	}
 	desktop_damage_whole_container(con);
 	container_floating_translate(con, lx - con->x, ly - con->y);
 	desktop_damage_whole_container(con);