How elements stack on top of one another depends on the z-index styles and the stacking contexts.
The first post above shows how to set an arbitrary z-index style to Kendo UI loading overlay:
However, if the overlay itself is placed inside a lower stacking context, it will not be able to cover elements from a higher stacking context. In this case you will also need to apply z-index styles to other ancestor elements, so that the stacking contexts' are rearranged.
If you need further assistance with this, please send an example, so that I can get familiar with the scenario and provide specific advice.
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