bug#19395: 25.0.50; Setting left fringe to 0 messes up window-width

[Eli Zaretskii <eliz@gnu.org>]

> From: Titus von der Malsburg <malsburg@posteo.de>
> Cc: 19395@debbugs.gnu.org
> Date: Tue, 16 Dec 2014 12:36:13 -0800
> 
> > The n+1st character is usurped for the continuation glyph.  This is
> > not a bug.
> 
> Sorry, but I don't understand that.  If (window-width) says there is
> space for 50 characters and I put 50 characters on a line, there
> shouldn't be a continuation character in the first place.

window-width doesn't report the number of characters a window's line
can hold without continuation.  It reports the window width in
character units.  When there's no fringe to display the continuation
bitmap, that width includes the space reserved for the continuation
glyph, which is the backslash character produced by the display engine
on the last column of the line.

> Also, I don't see why I should get different behaviour for different
> values of left-fringe.  When left-fringe is > 0, I get as many
> characters on a line as (window-width) reports.  But if left-fringe
> is 0, I get one character less on the line.

As long as the left fringe exists, we can display on it.  But once its
width is zero, it no longer exists, and so we need to reserve space to
display the continuation glyph "by other means".

> This behaviour doesn't seem to be consistent with the documentation
> of window-width.  Quote:
> 
>   The return value does not include any vertical dividers, fringes or
>   marginal areas, or scroll bars.

This doesn't say anything about continuation and truncation glyphs, so
I see no inconsistency here, perhaps just missing details.

> My understanding of this is that the fringes should not matter at
> all.  If window-with reports n, I should be able to fit n characters on
> a line irrespective of what the fringes are.

I tried to explain above why this interpretation is incorrect:
window-width does not return the number of characters of buffer text
that can be displayed on a line.

> Moreover, why is this specific to the left-fringe?  The value of
> right-fringe does not affect influence whether I can get
> (window-width) or (window-width) -1 characters on a line.

That's not what I see here.  Setting any of the fringes' width to zero
causes window-width to report 1 more character than you can put on a
line without going to a continuation line.  IOW, the "penalty" is
symmetrical, at least on my machine.