<p>This tests to make sure that a br isn't inserted into a tab span during an InsertLineBreak operation.  You can test for its existence with the DOM inspector or you can look at the render tree.</p>
<div id="div" contenteditable="true">
<div><span class="Apple-tab-span" style="white-space:pre">	</span>foo</div>
<div><span class="Apple-tab-span" style="white-space:pre">	</span>bar</div>
</div>

<script>
div = document.getElementById("div");
div.focus();
sel = window.getSelection();
sel.setPosition(div, 0);
sel.modify("extend", "forward", "paragraph");
document.execCommand("InsertLineBreak");
</script>