a765507c9d
Also: - Make Dismissable report when it starts squashing, since otherwise we don't invalidate the list early enough and it gets mad that it wasn't told one of its children had changed size. - Have Dismissable check that it gets removed after it's dismissed, to avoid having lots of redundant widgets around. - Start tracking the height of each child of a MixedViewport, so that we don't accumulate floating point errors when a child jiggles up and down. - Have _childOffsets reuse its storage space rather than newing up a new copy each time we reset the cache. - Avoid double-updating child sizes when handling mixed viewport invalidations.