I have a monitor running at 125% scaling (set in Windows Display settings). The radgrid width is calculated incorrectly. For some reason I always get a grid that is 503px wide (100% would be about 1500).
Both setting the scale to 100% and refreshing the page fixes it.
Moving the window to another monitor at 100% scaling and refreshing fixes it.
This looks to 100% be a bug in the way Telerik determines width when scaling is used.