This commit takes control of keyboard-based pagination away
from the browser, so that we can use the effective viewport
size as the amount to page, as well as keeping a little bit
of overlap from page to page. There had been issues with
pagination for a while, but the introduction of the always-open
composebox particularly aggravated the situation.
(imported from commit 45b9b7d5a6b322230c9d55e1be0b763dbce06e2e)