type
TForm1 = class(TForm)
private
procedure WMMoving(var Message: TWMMoving); message WM_MOVING;
public
{ Public declarations }
end;

var
Form1: TForm1;

implementation

{$R *.dfm}

{ TForm1 }

procedure TForm1.WMMoving(var Message: TWMMoving);
begin
Message.DragRect^.TopLeft:= Self.BoundsRect.TopLeft;
Message.DragRect^.BottomRight:= Self.BoundsRect.BottomRight;
inherited;
end;