Sending a message while scrolled up left it out of sight: the thread only
ever anchored its bottom, so nothing brought the view down. The column now
follows new messages while it sits at the bottom, stays put while it's
scrolled up, and always brings a message the user just sent into view.
While the thread sits above its bottom, "Jump to latest" floats over it.
Whether the thread is at its bottom is read from where the content ends
rather than from `scrollTop`, whose sign a reversed column reports
differently across engines. Content that grows after it lands (an image,
a wider window) is followed too, so the newest message never slips under
the composer.
