I've had this problem for a while--it is not new with 6.4. If I get certain combinations of tabs open in a panel, they toggle back and forth in some type of loop. I think that one tab decides it needs to go from title to icon because the bar is too long, then when it switches, there's enough room to expand one from icon to title...then it's too long again.
Attached is a short clip of it in action.
1667.PanelBug.mov