Bolão via tecla Shift | JavaScript e HTML: pratique lógica com desenhos, animações e um jogo

<!DOCTYPE html>
<html lang="pt-br">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <canvas width="600" height="400"></canvas>

    <script>
        let tela = document.querySelector("canvas");
        let pincel = tela.getContext("2d");

        pincel.fillStyle="grey";
        pincel.fillRect(0,0,600,400);
        pincel.fill();

        function gerarBolinha(evento){
            var x = evento.pageX - tela.offsetLeft;
            var y = evento.pageY - tela.offsetTop;

            if(evento.shiftKey){
                pincel.fillStyle="blue";
                pincel.beginPath();
                pincel.arc(x, y, 10+20, 0, 2 * 3.14);
                pincel.fill();
            } else {
                pincel.fillStyle="blue";
                pincel.beginPath();
                pincel.arc(x, y, 10, 0, 2 * 3.14);
                pincel.fill();
            }
        }

        tela.onclick = gerarBolinha;


    </script>

</body>
</html>