<!DOCTYPE html>
<html>
<head>
    <style>
        .box {
            margin: 50px;
            height: 150px;
            width: 150px;
        }

        .before {
            overflow-y: scroll;
            background-color: red;
        }

        .shadow {
            margin: 20px;
            height: 300px;
            width: 300px;
            background-color: silver;
            border: 1px solid black;
            -webkit-filter: drop-shadow(10px 10px 10px blue);
        }
    </style>

    <script>
        if (window.testRunner)
            testRunner.waitUntilDone();

        function doTest()
        {
            window.setTimeout(function() {
                document.querySelector(".before").classList.remove("before");
                if (window.testRunner)
                    testRunner.notifyDone();
            }, 0);
        }
        
        window.addEventListener('load', doTest, false);
    </script>
</head>

<body>

    <!-- You should not seen part of a blue shadow inside the gray box. -->
    <div class="shadow">
        <div class="before box"></div>
    </div>

</body>
</html>