DragDrop.info

  http://forum.lazarus.freepascal.org/index.php?topic=30263.0
  
  File "/usr/lib64/lazarus/lcl/include/treeview.inc"
  
  procedure TTreeNode.SetDropTarget(AValue: Boolean);
begin
 // if AValue=GetDropTarget then exit;
  if AValue then begin
    Include(FStates,nsDropHilited);
    if TreeView<>nil then
      TreeView.FLastDropTarget:=Self;
  end else begin
    Exclude(FStates,nsDropHilited);
    if TreeView<>nil then
      TreeView.FLastDropTarget:=nil;
  end;
  {if Value then TreeView_SelectDropTarget(Handle, ItemId)
  else if DropTarget then TreeView_SelectDropTarget(Handle, nil);}
end; 




procedure TCustomTreeView.DragOver(Source: TObject; X,Y: Integer;
  State: TDragState; var Accept: Boolean);
var
  Node: TTreeNode;


begin
  inherited DragOver(Source,X,Y,State,Accept);
  Node := GetNodeAt(X, Y);
  {$IFDEF VerboseDrag}
  DebugLn(['TCustomTreeView.DragOver ',Name,':',ClassName,' ',Node<>nil,' ',Node <> DropTarget,' ',Node = FLastDropTarget]);
  DebugLn(['TCustomTreeView.DragOver Source ',Source,':',Source.ClassName]);
  {$ENDIF}
 
  if (Node <> nil)
  and ((Node <> DropTarget) or (Node = FLastDropTarget)) then
  begin
    FLastDropTarget := nil;
    Node.DropTarget := True;
  end
  else if Node = nil then FLastDropTarget := nil;    //added this
end;