An upward reader scroll that left the newest message's end at the
viewport bottom still read as at tail, so new content pulled the reader
down past the composer pin. The message-end tail position now counts
only on a downward scroll. A channel-disabled Conversation pins the
newest message instead of a composer that cannot send. S11 and S12
visibility checks require the whole newest message whenever it fits
above the composer.
In page flow (200% zoom, short or narrow windows) the transcript now
follows with the composer's bottom at the viewport bottom and the newest
message above it. Absent or disabled composers keep the S11 pin. Narrow
Room composers stack in one column. S11's visibility check accepts the
newest message's end when it is taller than half the viewport.
jason.woltje