Yet another redisplay question

[Dmitry Antipov]

AFAICS redisplay engine does a lot of iterator movements backward to BEGV
or forward to ZV.  From the redisplay's point of view, how much of buffer
text beyond its visible boundaries (window-start) and (window-end) we
should process to be sure that the visible part is displayed correctly?
In particular, when we're looking for the beginning of the previous line,
or the end of the current line, why not limit the search within the
visible part of text?

Dmitry