Why does REFRESH MATERIALIZED VIEW block concurrent readers, and how do you avoid that?

A plain REFRESH truncates the existing storage and reruns the defining query from scratch, taking an ACCESS EXCLUSIVE lock for the duration — any query touching the view blocks until it finishes. REFRESH MATERIALIZED VIEW CONCURRENTLY avoids this by computing the new result into a temporary copy and diffing it in, but it requires a unique index on the materialized view first.