Update: I found the right file:
\public\legacy\themes\suite8\css\Dawn\style.css
If you change:
#detailpanel_report .list-view-rounded-corners{margin:0px}
to this:
#detailpanel_report .list-view-rounded-corners{margin:0px;overflow-x:scroll;}
Then do a R&R it works.
… almost… in one installation it works. In another, it only works with developer mode on. So, there’s still a cache file somewhere I’m not getting. I wish there was some easy way to just flush out all the old CSS.