I believe I’ve isolated my issue: containers can’t be tables, looks like they must be divs.
I created a pen which had two identical containers, except one was a table.
The container query fails on the table, but succeeds on the otherwise almost identical div.
the ultimate solution here was moving the container up to #list-area which is a div and that solved my issue!
Really appreciate you looking at this, because confirming it should work in themes (to the extent that it does) gave me that all-important extra impetus!