Du musst nur eine Zeile hinzufügen:
Delphi-Quellcode:
procedure TForm1.WMNCHitTest(var M: TWMNCHitTest);
begin
inherited;
if M.Result = htClient then
if ControlAtPos(ScreenToClient(SmallPointToPoint(M.Pos)), false) = nil then
M.Result := htCaption;
end;