procedure TDemoForm.TreeViewDragDrop(Sender, Source: TObject; X, Y: Integer);
var
  HT: THitTests;
  AttachMode: TNodeAttachMode;
  TargetNode: TTreeNode;
begin
  HT := TreeView.GetHitTestInfoAt(X, Y);
  TargetNode := TreeView.GetNodeAt(X, Y);
  if (htOnItem in HT) then
  begin
    AttachMode := naAddChild;
    TargetNode.Expand(False);
  end else if (htOnIndent in HT) then
    AttachMode := naInsert
  else if (htOnRight in HT) then
    AttachMode := naAdd
  else
    Exit;

  with Sender as TTreeView do
    Selected.MoveTo(TargetNode, AttachMode);
end;