I think this is caused by jquery-ui failing to load, which should have created the `resizeable` property on that element.
jquery-ui was updated from 1.12.1 to 1.13.0 in november (but that change wouldn't have been picked up until the jupyter notebook javascript was rebuilt recently to try and fix issues with libjs-marked). However, I haven't worked out _what_ broke yet. Ideas on a postcard.
