Im not using this particular feature (at present) but I came across this bug as I was testing form styling of the various form fields to match my site's design.
By process of elimination of the form fields that I had on the page at the time, I've identified the Drop Down field only when using the Enhanced Interface, as triggering the bug.
It's throwing a JS error: TypeError: a.browser is undefined
/wp-content/plugins/gravityforms/js/chosen.jquery.min.js?ver=1.7.2
Worth mentioning is that it also stopped all other JS on the page from functioning.