+static gboolean on_button_motion_event (GtkWidget *widget, GdkEventMotion *event,
+ game_t *game, cairo_t *cr)
+{
+ tile_t *tile;
+
+ tile = &game->deck.tiles[game->current_tile];
+
+ /* First, invalidate the region where the tile currently is. */
+ gtk_widget_queue_draw_area (widget, tile->x - 1, tile->y - 1, TILE_WIDTH + 1, TILE_HEIGHT + 2);
+
+ /* Then, move the tile */
+ tile->x = event->x - game->diff_x;
+ tile->y = event->y - game->diff_y;
+
+ /* Finally, invalidate the region where the tile is now. */
+ gtk_widget_queue_draw_area (widget, tile->x - 1, tile->y - 1, TILE_WIDTH + 1, TILE_HEIGHT + 2);
+
+ return TRUE;
+}
+