We have already fixed a problem related to the ViewportSize (similar to the described one) which we had reproduced when the following preconditions were available:
1. Window's SizeToContent="WidthAndHeight"
2. RadGridView is wrapped in ScrollViewer with HorizontalScrollBarVisibility="Auto" VerticalScrollBarVisibility="Auto" properties set.
3. RadGridView is bound to an empty itemsource
4. RadGridView's property ColumnsWidthMode =Fill
The fix will be available in our latest internal build(this Friday). But we still cannot be sure if this fix will fix the issues you have. That's why it would be best if it is possible to send us a small example where we can reproduce the problem.
the Telerik team