Why does Mail's Viewer list keep jumping to the top?

Thanks, @P. Phillips, for your screenshot. It prompts me to add that I'm using the classic View > Use Column Layout.

In my case, I have Sort By > Oldest Message on Top. Since Mail typically leaves the list positioned at the most recently selected message, and I'm typically looking at the most recent messages, the list should remain scrolled to the bottom of the list.

A few minutes ago, I saw the email from Apple Support announcing your reply to this discussion. I double-clicked the Viewer line, and the message body appeared in a new window. When I clicked the "view the full discussion" link, this page appeared in a new Safari window. At the same time, Mail redisplayed my lengthy Inbox in the Viewer window and positioned it at the top of the list (i.e., oldest messages). Perhaps that redisplay was prompted by a the arrival of a new email – I didn't notice.

I dragged the Viewer's scroll bar to the bottom and repeated opening the message and clicking the link. This time, the View list remained positioned at the end of the list.

Repositioning to the top of the list doesn't happen all the time. But it's often enough to be very annoying.

My dialogue with @P. Phillips has given me a new insight on this "jump to the top" behavior.

The times I observe Viewer list shift to the top if the list, it's always preceded by a very animated redisplay of the thousands of messages in my Inbox. That is, the list doesn't simply redisplay from the top. Instead, it's a rapidly changing list, with new lines appearing randomly in the window and existing lines moving up or down. After 5-15 seconds of animation, the list comes to rest at the top.

This raises new questions: Why does Mail need to spontaneously redisplay the whole Inbox? Why doesn't it simply redisplay from the top?

My Viewer list is sorted by ascending Date Received just like in the screenshot.

I watched more closely this time as I went from the reading the email announcement to viewing the new screenshot. The Viewer window remained positioned at the bottom. Deleting the 2-line announcement discussion also left the list positioned at the bottom. This may be a case of a "watched pot" because I know that filing or deleting the last email in the Viewer window has triggered a redisplay and repositioning to the top.