LayoutTests/intersection-observer/initial-observation.html

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

<script>
if (window.internals) {
    // Disable timers that trigger delayed flushes, since these will make
    // this test pass spuriously, even if IntersectionObserver never
    // schedules any rendering updates.
    internals.setSpeculativeTilingDelayDisabledForTesting(true);
    internals.disableTileSizeUpdateDelay();
}

window.onload = function() {
async_test((t) => {
    var observer = new IntersectionObserver(t.step_func_done(() => {
        observer.disconnect();
    }));
    requestAnimationFrame(() => {
        requestAnimationFrame(() => {
            setTimeout(() => {
                observer.observe(document.getElementById("target"));
            }, 0);
        });
    });
}, "An initial observation is fired even when nothing else triggers rendering");
};
</script>

<style>
#target {
  width: 100px;
  height: 100px;
  background-color: green;
}
</style>

<div id="target"></div>