I’m using the silverlight scrollbar and I’m finding that the mouse does not drag the scrollbar 1:1. When I drag downwards for instance I find the mouse cursor starts going down further than the scrollbar has..
edit: this issue occurs in IE8 but not Firefox 5.
Are you working with datasets or trying to display large, complex data within the object whatever the scrollbar is attached to?
If yes, try toggling UI virtualization and see if it changes anything.
If it doesn’t work it may be something to do with GPU acceleration. Just toggle GPU acceleration in both SL project and IE. Sometimes when CPU loads a lot while scrolling, it just doesn’t scroll correctly because of this, well, I call “lag”.