private
  procedure WMWINDOWPOSCHANGING (Var Msg: TWMWINDOWPOSCHANGING); message WM_WINDOWPOSCHANGING;
  public
  end;
...
procedure TfrMain.WMWINDOWPOSCHANGING(var Msg: TWMWINDOWPOSCHANGING);
const
  StickAt : Word = 10;
var
  rWorkArea: TRect;
begin
  SystemParametersInfo (SPI_GETWORKAREA, 0, @rWorkArea, 0);
  with rWorkArea, Msg.WindowPos^ do
  begin
  if x <= Left + StickAt then
  x := Left;
  if x + cx >= Right - StickAt then
  x := Right - cx;
  if y <= Top + StickAt then
  y := Top;
  if y + cy >= Bottom - StickAt then
  y := Bottom - cy;
  end;
  inherited;
end;