Re: "... the window start at a meaningless point within a line."

[Eli Zaretskii]

Btw, you said you ignore display and overlay strings for now, but
their existence raises a serious problem wrt your "exact window-start"
solution: what if a display string starts on one screen line and ends
on the next one, i.e. the continuation happens in the middle of a
display string?  In this case, you will not be able to force a
window-start on a buffer position that is "covered" by the display
string, so some part of the string will either be displayed twice, or
not at all.

This is also a problem with today's unmodified Follow mode: when such
a string is between two windows, the second portion of the string (the
one in the second window) is not displayed at all.