To reproduce:
- Add items to the tree view (scrollbar must appear)
- Add context menu item which adds a new node and puts it in edit mode.
- Add a new node with the context menu item, scroll to the bottom, add item again.
- Scroll up and add item one more time.

Workaround:
void treeView_EditorRequired(object sender, TreeNodeEditorRequiredEventArgs e)
{
    e.Editor = new TreeViewTextBoxEditor();
}