Hi Richard,

I tried to reproduce the reported behavior in the following example:
https://stackblitz.com/edit/angular-zzykw2?file=app%2Fapp.component.ts

Indeed, when we click a cell, the popup is displayed and if we scroll, the popup will be shown out of the boundaries of the grid. In order to avoid this behavior we can handle the anchorViewportLeave event as demonstrated in the following article:
https://www.telerik.com/kendo-angular-ui-develop/components/popup/closing/#toc-after-leaving-the-viewport

Let me know in case I am missing something or further assistance is required for this case. In case the issue is something else, could you provide some more relevant details.

I would also like to add, that you can open a private support ticket in case further discussion is needed for this case as the response time will be less, than the response time for a public feedback item.

Regards,
Svetlin
Progress Telerik