Since pinned rows and columns do not scroll, we should assert they are all contained within the viewport.
Edge case to also consider: If there are pinned rows/columns that consume the full extent of the viewport, and there are additional unpinned children we should probably also assert, since the additional children will never be visible to the user.

Update: Rather than asserting here, we should probably just surface a non-fatal warning of some kind. A good example of this is in Google sheets. If you freeze rows/columns beyond the window size, it is allowed, but a small dialog pops up to inform the user they cannot scroll anything in to view in that configuration.