The problem was that when I moved the main window to try to find the “extra update window” I moved it down. I didn’t realise it, but that caused the window to be minimised, which made it look like it was either crashing, or going off to do the update. Actually it killed the program. I finally worked out that I had to restart the computer in order to get it to show the “extra update” window again. Then I moved the main window sideways, and it worked. The bummer is that it then said my OS is out of date and Zwift won’t run after August 1st. The computer’s 7 years old and Apple have stopped supporting OS updates. I guess I need a new computer now. 