Fix moving child to first position in tree data (#9761)

Resolves #9760

server/src/main/java/com/vaadin/data/TreeData.java

@@ -438,7 +438,8 @@ public class TreeData<T> implements Serializable {
      * @param item
      *         the item to be moved
      * @param sibling
-     *         the item after which the moved item will be located
+     *         the item after which the moved item will be located, or {@code
+     *         null} to move item to first position
      * @since 8.1
      */
     public void moveAfterSibling(T item, T sibling) {
@@ -447,26 +448,34 @@ public class TreeData<T> implements Serializable {
                     "Item '" + item + "' not in the hierarchy");
         }
 
-        if (!contains(sibling)) {
-            throw new IllegalArgumentException(
-                    "Item '" + sibling + "' not in the hierarchy");
+        if (sibling == null) {
+            List<T> children = itemToWrapperMap.get(getParent(item))
+                    .getChildren();
+
+            // Move item to first position
+            children.remove(item);
+            children.add(0, item);
+        } else {
+            if (!contains(sibling)) {
+                throw new IllegalArgumentException(
+                        "Item '" + sibling + "' not in the hierarchy");
+            }
+
+            T parent = itemToWrapperMap.get(item).getParent();
+
+            if (!Objects.equals(parent,
+                    itemToWrapperMap.get(sibling).getParent())) {
+                throw new IllegalArgumentException(
+                        "Items '" + item + "' and '" + sibling
+                                + "' don't have the same parent");
+            }
+
+            List<T> children = itemToWrapperMap.get(parent).getChildren();
+
+            // Move item to the position after the sibling
+            children.remove(item);
+            children.add(children.indexOf(sibling) + 1, item);
         }
-
-        T parent = itemToWrapperMap.get(item).getParent();
-
-        if (!Objects
-                .equals(parent, itemToWrapperMap.get(sibling).getParent())) {
-            throw new IllegalArgumentException(
-                    "Items '" + item + "' and '" + sibling
-                            + "' don't have the same parent");
-        }
-
-        List<T> children = itemToWrapperMap.get(parent).getChildren();
-
-        // Move item to the position after the sibling
-        // If sibling is null, item is moved to the first position
-        children.remove(item);
-        children.add(children.indexOf(sibling) + 1, item);
     }
 
     /**

server/src/test/java/com/vaadin/data/provider/TreeDataProviderTest.java

@@ -123,6 +123,19 @@ public class TreeDataProviderTest
         data.moveAfterSibling(root0, null);
         Assert.assertEquals(root0, data.getRootItems().get(0));
         Assert.assertEquals(root9, data.getRootItems().get(9));
+
+        StrBean child0 = data.getChildren(root0).get(0);
+        StrBean child2 = data.getChildren(root0).get(2);
+
+        // Move first child to different position
+        data.moveAfterSibling(child0, child2);
+        Assert.assertEquals(2, data.getChildren(root0).indexOf(child0));
+        Assert.assertEquals(1, data.getChildren(root0).indexOf(child2));
+
+        // Move child back to first position
+        data.moveAfterSibling(child0, null);
+        Assert.assertEquals(0, data.getChildren(root0).indexOf(child0));
+        Assert.assertEquals(2, data.getChildren(root0).indexOf(child2));
     }
 
     @Test(expected = IllegalArgumentException.class)