Как узнать, какое окно закрывает форму?

//Find windows that may cover another window.
Var
  hW: HWnd;
  r: TRect;
begin
  hw := Handle;
  While IsWindow(hw) Do Begin
    hw := GetWindow( hw, GW_HWNDPREV );
    If IsWindowVisible(hw) and not IsIconic( hw ) Then Begin
      ... use GetWindowRect( hw, r ) to get candidate windows
      rect and check if it intersects the forms BoundsRects via
      IntersectRect
    End;
  End;
end;