I noticed that when you mouse over a thread to view the description in firefox 3.6.3, the box always appears above the thread. However for threads near or at the top of the screen this puts the description box completely off the screen. Perhaps it should realize the box isn't going to fit above the thread title and pop up below the thread instead.
Agree. I experience this problem too. It's a bit irritating because you have to scroll page up a little and then summon tooltip again. I have tested this on latest stable Google Chrome - the same problem.
There are technical issues that make this very challenging (namely the dynamic loading). Additionally, I'm not sure if putting it at the bottom is helpful because of navigation flow issues (reading the next thread). So partially can't fix, partially as designed.