>When I click on the "New messages during this session icon" in the bottom left frame, the list (map) of messages since last log on (top right frame) is reset to display only the new messages this session & the count of new messages this session is reset to 0. I lose the list of messages between last log on & the current time.

This is exactly what this option is about. If you click on the icon, it is as is you were entering the forum. Which means, you'll get all new messages. Of course, don't click on it, if you haven't finished reading the new ones which are already shown on the upper right frame. However, if you want to get new messages without refreshing those, just click on the link under the icon instead. You'll then get them in the lower right frame.