dropPosition is calculated incorrectly when dragging within the same parent

[Buskervil]

When using Tree with draggable enabled, the dropPosition property in the onDrop event returns an incorrect index if an element is dragged downward within the same parent.

Steps to Reproduce:

Create a tree with nested nodes.

Enable draggable and handle onDrop.

Drag a node downward inside the same parent (without changing nesting level).

Check event.dropPosition – it will be +1 higher than the actual index.

Expected Behavior:
dropPosition should accurately reflect the drop position.

Current Behavior:
If a node is dragged after another node within the same parent, dropPosition is incremented by 1 (e.g., returns 2 instead of the actual index 1).

Additional Context:

The event.node.children property is sometimes missing, making manual position calculation harder.

Observed in antd@5.24.8, but might affect other versions.

import React, { useState } from 'react';
import { Tree } from 'antd';
import type { TreeDataNode, TreeProps } from 'antd';

const initialData: TreeDataNode[] = [
  {
    title: 'Parent',
    key: '0',
    children: [
      { title: 'Child 1', key: '0-0' },
      { title: 'Child 2', key: '0-1' },
      { title: 'Child 3', key: '0-2' },
    ],
  },
];

const App: React.FC = () => {
  const [gData, setGData] = useState(initialData);

  const onDrop: TreeProps['onDrop'] = (info) => {
    console.log('Actual drop position:', info.dropPosition); // Will be 1 greater than the actual position when moving downward
  };

  return (
    <Tree
      draggable
      blockNode
      onDrop={onDrop}
      treeData={gData}
    />
  );
};

export default App;