Popup containers are normally placed at the end of the body element, because this is the only way to ensure that they will actually be on top of everything else and not sink inside some lower stacking context.
If you manually change some content on your page, including the Grid, you will need to clean up the filter menus as well.
In the future, we may implement a dispose method for the Grid, so that when you call it, the Grid will know what to look for on the page and what to clean, before you remove it from the DOM.
the Telerik team
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