loc_sort_order mode, end of range on one end locks both ends

[jrochkind]

Say you are in loc_sort_order mode. Say you happen to actually be near the beginning or end of the entire LC range.

You scroll up, and the server side script returns a start: "-1", stackview notices this means "end of things", and does not let you scroll up any further beyond this, all is well.

But when you go to scroll down, you also can't scroll down past the initial load, even though end-of-range hasn't been reached there. Reaching end-of-range on one end locks up the whole thing, no more content can be loaded on the other end either.

This won't come up in practice too much, because you are unlikely actually reach the end of a lengthy call number range. But it's still pesky.

I haven't been able to make much headway in debugging and understanding the related code here. Any hints for me on what it is where that is disabling scrolling when you reach end of range?