Currently we send prefetch signals during onBind(), based on the page being touched. Two problems:

if all items are filtered during initial load, we won't fetch more data at all. Note**
if enough items are filtered, we may be fulfilling prefetchDistance in the unfiltered space, but have run out of data in the UI side.

Note**: generally especially aggressive filtering should be done at the PagingSource level, e.g. with a filtering Room query, so that paging doesn't go through the fetch process and notifying network state for empty pages.

See also b/149482600#4, when someone hit 1)