There seems to be an issue with certain grid types in IE8. Here is an official Ext JS example,
The problem occurs when you scroll down and try clicking on a row, the scrollbar seems to jump to a random position on the page, I have been playing with this for a while and cannot see any pattern in the behavior. Maybe it is trying to center the selection and has incorrect Y coordinates after scrolling? Either way this is certainly not desired behavior. I have not tested other versions of IE but Firefox and Chrome do handle it properly.
Also, I realize the example page is for Ext JS 3.0, however the problem exists in the 2 branch as well (which is what I am using)