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.
jason.woltje
0e34b30770