Woohoo. Found the culprit:
<script type="text/javascript" src="https://stackoverflow.com/scripts/jquery/132/jquery-1.4.2.min.js"></script>
<script type="text/javascript" src="/scripts/jquery/132/jquery-ui-1.8.custom.min.js"></script>
<script type="text/javascript" src="/scripts/jquery/100325/jquery.dimensions.js"></script>
<script type="text/javascript" src="/scripts/jquery/100325/jquery.tooltip.js"></script>
Don’t include jquery.dimensions.js. I assume it’s already in jquery-ui.js … anyway, it fixed my problem.
For latest jqueryUI you now need to include jquery.ui.position.js