LayoutTests/http/tests/intersection-observer/intermediate-frame-changes.html

<!DOCTYPE html>
<script src="../resources/testharness.js"></script>
<script src="../resources/testharnessreport.js"></script>

<style>
iframe {
  width: 300px;
  height: 300px;
}
</style>
<iframe id="intermediateFrame" src="resources/intermediate-frame.html"></iframe>

<script>
var entries = [];
var iframe = document.getElementById("intermediateFrame");
iframe.onload = function() {
    var nestedFrame = iframe.contentDocument.getElementById("nestedFrame");
    var scroller = iframe.contentDocument.getElementById("scroller");
    var target = nestedFrame.contentDocument.getElementById("target");

    async_test(function(t) {
        new IntersectionObserver(function(changes) {
            entries = entries.concat(changes);
            if (entries.length == 1) {
                assert_false(entries[0].isIntersecting);
                scroller.scrollTop = 250;
            } else if (entries.length == 2) {
                assert_true(entries[1].isIntersecting);
                scroller.scrollTop = 0;
            } else {
                assert_false(entries[2].isIntersecting);
                t.done();
            }
        }).observe(target);
    }, "Intersection observations are updated when an intermediate frame changes");
};
</script>