Using Shift + Up/Down or Shift + PageUp/PageDown (and probably other variants of selecting several messages) doesn't update the focus rectangle correctly - the focus rectangle doesn't get removed from the previous message correctly. Only one message should have the focus rectangle.
Doing some investigation I have noticed something which _might_ be related: Left mouseclicking on a message, let's say the fifth message in the list and then shift + left mouseclicking on message number 8, correctly marks 5,6,7,8 as selected with focus on 8, but if I after this use keyboard down selecting message number 10, and again after this shift + mouseclick message number 12, I would expect only 10, 11 and 12 to be selected with focus on 12 - What instead happens is that 5 to 12 is marked, with focus on 12. This means that having only one message selected, and clicking on the message just below is different from pressing keyboard down.
Yep, it's minorly annoying to have to reach for the rat :). Been around for a while, not new to 3.12.