In FormShow:

var


RecS, RecL: TRect;
begin
  RecS := Rect(Screen.Width, Screen.Height, Screen.Width, Screen.Height);
  RecL := ThisForm.BoundsRect;
  DrawAnimatedRects(GetDesktopWindow, IDANI_CAPTION, RecS, RecL);
  { ... }
end;

In FormHide:

var
  RecS, RecL: TRect;
begin
  HideTimer.Enabled := False;
  RecS := Rect(Screen.Width, Screen.Height, Screen.Width, Screen.Height);
  RecL := ThisForm.BoundsRect;
  DrawAnimatedRects(GetDesktopWindow, IDANI_CAPTION, RecL, RecS);
end;