Don't allow items outside grid

src/Grid.vue

@@ -44,6 +44,10 @@ export default {
   },
   methods: {
     positionAllowed(x, y, key) {
+      if (x < 0 || y < 0) return false;
+      if (x > this.cols) return false;
+      if (y > this.rows) return false;
+
       for (let item of this.items) {
         if (key === item.key) continue;
         if (x >= item.x && x < item.x + item.w) {