Instead of using a window of a fixed number of events, unpaginate based on the distance of the viewport from the end of the scroll range.
The ScrollPanel uses the scrollTokens to convey to its parent (the TimelinePanel, in this case) the point to unpaginate up to. The TimelinePanel then takes a chunk of events off the front or back of `this.state.events` using `timelineWindow.unpaginate`.
Fixes https://github.com/vector-im/vector-web/issues/2020
The 'should load new events even if you are scrolled up' test was sometimes
failing. It turned out that pagination *sometimes* wasn't starting soon enough
after setting the scrollTop, and awaitPaginationCompletion was incorrectly
believing it to have already been and gone.
Add an awaitScroll to make sure that we wait long enough for the pagination to
begin.
* Actually test what we were supposed to be testing (viz, that we can paginate
back down after we hit the top of the room)
* Make the cap on the number of events we show in the timeline a configurable
property, so that we can set it in the test
* Use a smaller cap in the test, so that we have to do less paginating to hit
the cap, to make the test run quicker.
* add some more logging so that we can see how far it's got if it gets stuck.
Sometimes it fails because awaitScroll() on :277 isn't resolving because onScroll isn't firing. We need to know if this is because we aren't changing scrollTop
Luke Barnard