This is due to some kind of bug in ListView that never resettles
properly for bottom-to-top views. This can arise when the pinned message
is loaded (because that squishes the view) or the window is resized
(because that also resizes the view.)
We can work around it by assuming the following:
1. The RoomPage knows the window is resizing because it gets its height
changed before TimelineView.
2. The first height change can be a marker to position the view at the
beginning.
This fixes the issue for me, I did the following in order to test this:
* Switch between many rooms, especially ones with a pinned message. Now
all of them start at the bottom as they should.
* Resize the window, ensure that if you scrolled it stays around that
position - otherwise it sticks at the bottom.
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