This is a bug in Bootstrap that was resolved here: https://github.com/iainvdw/bootstrap/commit/c786f454c9175cce5bcf9a7074a3e206533b3d1a
However it doesn't seem it is included with the 3.1.1 version used on your site.
To do it manually, replace this line:
offsetBottom != null && (position.top + this.$element.height() >= scrollHeight - offsetBottom) ? 'bottom' :
With this:
offsetBottom != null && (Math.ceil(position.top) + this.$element.height() >= scrollHeight - offsetBottom) ? 'bottom' :