Unfortunately, it's not possible to stack two sticky elements that are inside different containers:
https://stackoverflow.com/questions/54689034/pure-css-multiple-stacked-position-sticky
So instead I've moved them under the same container. The .form-actions needs to cover up some of the table border.
I don't like the deep nesting of markup or class naming.. pls suggest if you have better ideas!
c3e513e457