In looking at your online examples (and playing with it myself) I have found the grid renders rather strange in Firefox 3.0.5 and Safari 3.2.1 Specifically the headers and filter section stop short of the right side of the grid under Firefox and end where the scollbar starts. So there is a 17px gap that shows the grid background color in the upper right corner.
In safari it looks close, the grid headers and filter extend to the right side of the grid, but where the scrollbar starts, they are 1px off vertically.
Example: http://demos.telerik.com/aspnet-ajax/Grid/Examples/Styles/CustomSkin/DefaultCS.aspx
Google Chrome and IE seem to look as expected (though the bottom border of the filter menu doesn't extend do the end of the grid, it stops at the scrollbar)
In safari it looks close, the grid headers and filter extend to the right side of the grid, but where the scrollbar starts, they are 1px off vertically.
Example: http://demos.telerik.com/aspnet-ajax/Grid/Examples/Styles/CustomSkin/DefaultCS.aspx
Google Chrome and IE seem to look as expected (though the bottom border of the filter menu doesn't extend do the end of the grid, it stops at the scrollbar)