Some time ago, I noticed the scroll bars on the mailbox window of Apple Mail had disappeared. One moment they were there and the next, so it seemed they weren't. Sometimes they came back for a few moments and if you were quick with the mouse or the trackpad, you could grab them. But sometimes they resisted appearing completely. Now I have a lot of mailboxes in that window, collecting mail filtered by lots of rules. It's supposed to make me read only the ones that relate to work, and leave the fun ones for distraction later.
But I never had time to search to find out what had changed, since it wasn't me. Today, in my traditional New Year effort to fix all the computer things that are irritating me, I finally googled and found the solution. It seems that the scroll bars have vanished on all the apps, because Apple decided we didn't need them all the time. It's a setting in System Preferences under General. If you set the scrolling to "always" they don't disappear.
But it seems Apple has decided we don't need arrows on the scroll bar. I can live with that.