seatop_move_floating: make container respect pointer constraint

sway/input/seatop_move_floating.c

@@ -6,6 +6,7 @@
 
 struct seatop_move_floating_event {
 	struct sway_container *con;
+	double dx, dy; // cursor offset in container
 };
 
 static void handle_button(struct sway_seat *seat, uint32_t time_msec,
@@ -25,8 +26,9 @@ static void handle_button(struct sway_seat *seat, uint32_t time_msec,
 static void handle_motion(struct sway_seat *seat, uint32_t time_msec,
 		double dx, double dy) {
 	struct seatop_move_floating_event *e = seat->seatop_data;
+	struct wlr_cursor *cursor = seat->cursor->cursor;
 	desktop_damage_whole_container(e->con);
-	container_floating_translate(e->con, dx, dy);
+	container_floating_move_to(e->con, cursor->x - e->dx, cursor->y - e->dy);
 	desktop_damage_whole_container(e->con);
 }
 
@@ -47,18 +49,21 @@ void seatop_begin_move_floating(struct sway_seat *seat,
 		struct sway_container *con) {
 	seatop_end(seat);
 
+	struct sway_cursor *cursor = seat->cursor;
 	struct seatop_move_floating_event *e =
 		calloc(1, sizeof(struct seatop_move_floating_event));
 	if (!e) {
 		return;
 	}
 	e->con = con;
+	e->dx = cursor->cursor->x - con->x;
+	e->dy = cursor->cursor->y - con->y;
 
 	seat->seatop_impl = &seatop_impl;
 	seat->seatop_data = e;
 
 	container_raise_floating(con);
 
-	cursor_set_image(seat->cursor, "grab", NULL);
+	cursor_set_image(cursor, "grab", NULL);
 	wlr_seat_pointer_clear_focus(seat->wlr_seat);
 }