I found out that it was an experimental setting that caused the problem: "Redesigned bookmarks page
New design for bookmarks page supporting dark theme "
That is a misleading explanation. The dark theme only changes the colours, but not the function. Why anyone would want an interface like that is a complete mystery to me.
There is an experimental setting called "Redesigned bookmarks page" which is explained as being for the dark theme. That is what wrecked the bookmark manager interface. The dark theme with that setting makes no difference as to how it works. That just changes the colours. Why be so misleading about that setting?