When there are just enough messages to put one listing under the seperator bar between the pane containing the list of messages and the pane contining the message that is selected, then theseperator bar keeps moving down and back up the height of one message listing. This continues until one listing is removed from the list of messages by moving the message to the trash or marking it deleted and compacting the folder. The messages are on an IMAP server.
It sounds pretty bad but I can not reproduce it. In case when one message does not fit Thread pane, scroll bar appears. No horizontal splitter jumps.
Notice in 0.98 how the seperator bar moves down when you move from an imap server folder that has no messages to one that has enough messages to create a scrollbar. It looks like about twice the thickness of the seperator. On my system, when I get exactly 9 messages, the messae that is displayed begins to redisplay continuously (flicker) and very shortly the seperator bar starts to jump from the position where it would be if there were no scrollbar and where it would be if there was a scrollbar and the scrollbar displays and dissapears.
resolving as dup of bug 121583 (by way of bug 124530 -> bug 122153) *** This bug has been marked as a duplicate of 121583 ***