Why does the first link cause iframe-in-iframe, but not the second link?

child.html

<h2>Child</h2>
<script>
    parent.document.getElementById("link1").onclick = function() {
        location.replace("#foo");
        return false;
    };

    parent.document.getElementById("link2").onclick = function() {
        setTimeout(function() {
            location.replace("#foo");
        }, 0);
        return false;
    };
</script>

index.html

<h1>Parent</h1>
<a id="link1" href="#">Click</a>
<a id="link2" href="#">Click</a>
<iframe src="child.html">
</iframe>

Explanation here and here.