Eliminates the visible push-down/snap-back when the pagination loading
skeleton appears at the top of the message list. Achieved by rendering a
constant-height (~200px) top-of-list slot above messages whenever
hasMore===true; the skeleton's grey-bar contents toggle inside that slot
rather than the slot itself mounting/unmounting. Companion changes:
load-more trigger raised to fire before the slot enters the viewport with
an iOS Safari rubber-band guard, prepend scroll-restore math corrected to
handle non-zero prevScrollTop, useDelayedLoading threshold lowered to
50ms for pagination only.
Jannis Braun
bc286c2750