dragdrop-container: fix max-children logic issue

* Actually check if the dragged is already children to be sure.
* Fix situation for canAccept test in the onDrop method when
  onDragOver already denied the element from entering.

src/dragdrop-container.js

@@ -163,7 +163,7 @@ export default class DragDropContainer extends HTMLElement {
             return;
         }
 
-        if (!this.canAcceptChild(dragged, true)) {
+        if (!this.canAcceptChild(dragged, this.isAncestorOf(dragged))) {
             if (dragged instanceof DragDropChild) {
                 dragged.bounce();
             }
@@ -244,6 +244,24 @@ export default class DragDropContainer extends HTMLElement {
             this.#maxChildren = Number.parseInt(newValue);
         }
     }
+
+    /**
+     * Check if this child is the ancestor of the given element.
+     *
+     * @param {HTMLElement} element
+     *
+     * @returns {boolean}
+     */
+    isAncestorOf(element) {
+        let parent = element.parentElement;
+        while (parent) {
+            if (parent === this) {
+                return true;
+            }
+            parent = parent.parentElement;
+        }
+        return false;
+    }
 }
 
 customElements.define('dragdrop-container', DragDropContainer);