Рисование квадрата мышкой

private
  { Private declarations }
  AnchorX, AnchorY,
  CurX, CurY: Integer;
  Bounding: Boolean;
end;

implementation

procedure TForm1.FormMouseDown(Sender: TObject; Button: TMouseButton;
  Shift: TShiftState; X, Y: Integer);
begin
  AnchorX := X;
  CurX := X;
  AnchorY  := Y;
   CurY := Y;
  Bounding := True;
end;

procedure TForm1.FormMouseMove(Sender: TObject; Shift: TShiftState; X,
  Y: Integer);
begin
  if Bounding then
  begin
    Canvas.Pen.Mode := pmNot;
    Canvas.Pen.Width := 2;
    Canvas.Brush.Style := bsClear;
    Canvas.Rectangle(AnchorX, AnchorY, CurX, CurY);
    CurX := X;
    CurY := Y;
    Canvas.Rectangle(AnchorX, AnchorY, CurX, CurY);
  end;
end;

procedure TForm1.FormMouseUp(Sender: TObject; Button: TMouseButton;
  Shift: TShiftState; X, Y: Integer);
begin
  if Bounding then
  begin
    Bounding := False;
    Canvas.Pen.Mode := pmNot;
    Canvas.Brush.Style := bsClear;
    Canvas.Rectangle(AnchorX, AnchorY, CurX, CurY);
  end;
end;