Usa el mensaje WM_WINDOWPOSCHANGING:
Código Delphi
[-]
type
TForm1 = class(TForm)
private
procedure WMWindowPosChanging(var Msg: TWMWindowPosChanging);
message WM_WINDOWPOSCHANGING;
end;
implementation
procedure TForm1.WMWindowPosChanging(var Msg: TWMWindowPosChanging);
begin
inherited;
end;
// Saludos