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

[Alan Mackenzie <acm@muc.de>]

Hello, Eli.

On Fri, Oct 16, 2015 at 08:35:15PM +0300, Eli Zaretskii wrote:
> > 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.

Without maybe_move_to_exact_bol, vertical-motion, when it lands near
the top of the window, leaves the cursor not at column 0 on the screen.

So, for example, with point at C, unmodified (vertical-motion -1) takes
us to A2, when we want to go to L3.

> Thanks.

-- 
Alan Mackenzie (Nuremberg, Germany).