Thanks; using Time Machine, I’ve reverted to 5.4.2 without difficulty and that (continues) to run well.
5.5 fell over when I first tried to open an earlier file, created and saved with 5.4, which wasn’t a great start. When I did open a file, I noticed that there was problem with rendering dimensions: they appeared odd, kind of blacked out, as one might see text redacted from a legal document. Is that the layout problem that you mention? (NB it was only the dimensions that were blacked out, although I can’t say I spent long assessing).
The falling over of 5.5 was surprising, as the previous versions have been stable. In fact, astonishingly stable when one considers what is being accomplished. Despite what I trust is a passing hiccup, I remain hugely impressed with Rhino.