This is an addendum to my previous post about this issue. I've never had this problem before, despite having multiple tabs open in individual Safari windows for at least the last ten years. So, I spent several hours on Thursday chatting online to an Apple advisor, who suggested all the possible fixes mentioned here, as well as completely reinstalling macOS in Recovery. None of those solutions worked, so the next advisor escalated the problem to a more senior colleague. I sent him a screenshot QuickTime file of the problem, but he'd never seen it before and could not replicate it on his machine. He suggested various changes to Safari Settings and System Settings that didn't resolve the problem and finally asked me to create a test user account, log out and then log back in to that test account. When I did that the problem did not exhibit itself on the new Safari window with the Separate tabs layout. His conclusion was that my System Settings were causing the problem and he suggested I migrate all my data to the new account and use that instead. I'm loathe to do that as, apart from the time and effort involved, there is no guarantee that the problem won't just reappear when I customise the Safari and System settings in the new account. I also use Firefox with multiple tabs on the same MacBook Pro and I'm not experiencing the same problem with Firefox. But, of course, Safari is part of macOS, whereas Firefox isn't. If it's not fixed in a future upgrade I may just move to Chrome or Opera, instead of Safari.
