A client, who has a site built in a builder-type theme, contacted me with an interesting problem. " "Categories" is there, but when I click the arrow to get the list, there isn’t one."
I made a test and had the same problem. If you click on the Categories block, about 1/4" of a panel shows, but not the whole panel to show a list of Categories. I was able to bring the rest of the panel to the front by moving the mouse pointer a bit after clicking or holding my mouse over the panel for several seconds.
I tested in posts that already had Categories attached, and the panel works fine.
The front side of this site is very slow because of poor design decisions, but I have not seen problems on the Dashboard side before.