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

[Alan Mackenzie <acm@muc.de>]

Hello, Eli.

On Mon, Oct 19, 2015 at 07:15:22PM +0000, Alan Mackenzie wrote:
> On Mon, Oct 19, 2015 at 04:27:38PM +0300, Eli Zaretskii wrote:
> > > Date: Mon, 19 Oct 2015 13:11:58 +0000
> > > Cc: emacs-devel@gnu.org
> > > From: Alan Mackenzie <acm@muc.de>

> > > > > I'm only really looking at the problem of getting to the correct
> > > > > beginning of line.  Moving to a specific column (with a cons argument to
> > > > > vertical-motion) seems to be less of a problem.

> > > > If that's in a continuation line, you have the same problem: the
> > > > display engine and the move_it_* functions count coordinates and
> > > > columns from their BOL, not from your "actual" BOLs.

> > > That's not what I see.  Having set `it' in Fvertical_motion to an
> > > iterator based at WS, the column movement remains based on `it', i.e.
> > > based on "actual" BOLs.

> > When move_it_by_lines returns, you are on the goal line, at zero X
> > coordinate.  That zero is in "xdisp coordinates".

> OK, I see it.  There's a "move_by_lines (it, -1)" near the end of
> maybe_move_to_exact_bol.  Usually, this would be OK, since we'd be
> dealing with "common" BOLs rather than "actual" BOLs, but there might be
> a borderline case where `it' would need to go up to the last "actual"
> BOL.  I'll need to look at this.

This "move_by_lines (it, -1)" isn't a problem, since the condition for
it occurring ensures it is only executed when starting from a "common"
BOL later than the first common BOL.

-- 
Alan Mackenzie (Nuremberg, Germany).