Scrolling beyond the page size in an infinite scoll grid (using the scrollbar) gets stuck when the loadMask appears. I've seen this behavior only in IE - tried with IE8 and IE9. Other browsers allow the user to pull the scroll bar all the way down irrespective of whether the loadMask is shown - as it should be.

This is also reproducible in the example for Infinite scroll grid at
http://dev.sencha.com/deploy/ext-4.0...te-scroll.html

Workaround:
To scroll beyond this you could release the scrollbar and try to pull it again until you hit the next page buffer load. This needs to be fixed to have a seamless scroll experience on IE.