The issue that you encounter is not related to the z-index of the controls but rather to the visibility property that is assigned to the different elements of the RadGrid control. You will need to get the correct css class selector and overwrite it by setting visibility property to visible
(you can use FireBug
tool of FireFox to get the correct css class selector). If this does not help I would ask you a live url that I can test locally and help you out in your particular case.
the Telerik team
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