Form layouts with .oo-ui-panelLayout-framed class currently have a box-shadow applied (0 0.15em 0 0 rgba(0,0,0,0.15)). Example: https://en.wikipedia.org/wiki/Special:MIMESearch
Following our current design M101, box-shadows as visual indicator are limited only to layers with higher z-index.

Easy way out would be to only remove the box-shadow and leaving the border around as is. That's similar to the former, non-OOjs UI fieldset layout like in https://en.wikipedia.org/wiki/Special:ListUsers

But I wonder, if we could remove the box-shadow and emphasize the connection of the fieldset to below appearing content even further…