added gridstack

src/main/java/com/example/components/GridStackItem.java

@@ -27,7 +27,11 @@ public class GridStackItem extends Div {
         getElement().setAttribute("gs-h", String.valueOf(h));
 
         this.content.addClassName("grid-stack-item-content");
-        add(this.content);
+        // Attach the wrapper div directly through the element API — calling
+        // the overridden add(Component...) below would route it through
+        // `content.add(...)`, making the wrapper try to add itself as its
+        // own child.
+        getElement().appendChild(this.content.getElement());
         this.content.add(content);
     }
 
@@ -80,7 +84,7 @@ public class GridStackItem extends Div {
     }
 
     /** Snapshot of a widget's grid placement, as reported by the client after a
-     *  drag/resize (see {@link GridStackLayout#onLayoutChange(String)}). */
+     *  drag/resize. */
     public record Position(String id, int x, int y, int w, int h) {
     }
 }