custom-scrollbar-inactive-pseudo.html

<!DOCTYPE HTML>
<style>
::-webkit-scrollbar {
    width: 12px;
}
::-webkit-scrollbar-track {
    box-shadow: inset 0 0 6px blue;
}
::-webkit-scrollbar-track:window-inactive {
    box-shadow: inset 0 0 6px red;
}
</style>
<script src="../resources/user-gesture-utils.js"></script>
<script>

function run() {
  if (window.testRunner) {
    testRunner.waitUntilDone();
    testRunner.setPopupBlockingEnabled(false);
  }

  var win = window.open('about:blank');
  if (window.testRunner) {
    window.addEventListener('blur', () => {
      testRunner.notifyDone();
    });
  }
  focusWithUserGesture(win);
}
</script>
<body onload="run()" style="overflow: scroll;"></body>

chromium/third_party/blink/web_tests/scrollbars/custom-scrollbar-inactive-pseudo.html