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

[Eli Zaretskii <eliz@gnu.org>]

> Date: Fri, 16 Oct 2015 09:55:35 +0000
> Cc: emacs-devel@gnu.org
> From: Alan Mackenzie <acm@muc.de>
> 
>   B---WS-----------------A---L2-------------A2---L3---------\nC-------C2---
> 
> When Fvertical_motion runs, it first sets its struct it "it" to a BOL, then
> possibly moves it to the appropriate column.  Between these two actions,
> maybe_move_to_exact_bol reseats `it' from an xdisp BOL to an exact BOL, or
> sometimes from a common BOL to an earlier or later BOL.

But why do you need to do that?  IOW, what problem does
maybe_move_to_exact_bol solve, and how that problem is related to what
Fvertical_motion does?

Perhaps start by explaining what problems you discovered in
Fvertical_motion that required you to make those changes.

Thanks.