type

 TWmMoving = record

  Msg: Cardinal;

  fwSide: Cardinal;

  lpRect: PRect;

  Result: Integer;

 end;

// Add a handler to your forms private section:

procedure WMMoving(var msg: TWMMoving); message WM_MOVING;

// Implement it as

 procedure TFormX.WMMoving(var msg: TWMMoving);

 var

  r: TRect;

 begin

  r := Screen.WorkareaRect;

  // compare the new form bounds in msg.lpRect^ with r and modify it if

  // necessary

  if msg.lprect^.left < r.left then

  OffsetRect(msg.lprect^, r.left - msg.lprect^.left, 0);

  if msg.lprect^.top < r.top then

  OffsetRect(msg.lprect^, 0, r.top - msg.lprect^.top);

  if msg.lprect^.right > r.right then

  OffsetRect(msg.lprect^, r.right - msg.lprect^.right, 0);

  if msg.lprect^.bottom > r.bottom then

  OffsetRect(msg.lprect^, 0, r.bottom - msg.lprect^.bottom);

  inherited;

 end;