IssueTracker

We'd like to avoid insertSeparators reliance on filtering empty pages to simplify rest of pageIndex handling internally.

One idea for this is to have a separate internal Page type for separators that includes which source page indices it's dependent on, and if any of those are removed we can also remove the separator page.

Another implication of this is that originalPageOffsetFirst / Last in ViewportHint is incorrect when the last / first page is empty due and gets filtered out by insertSeparators.