bug#19300: 24.4.51; visual-line-mode messes up after-string rendering when it spans all window columns

[Eli Zaretskii <eliz@gnu.org>]

> Date: Mon, 08 Dec 2014 20:47:02 +0200
> From: Dmitry Gutov <dgutov@yandex.ru>
> CC: 19300@debbugs.gnu.org
> 
> >> But I can open a different window, enable visual-line-mode, take its
> >> (window-width), insert that numbers of characters on one line and not
> >> see them wrapped.
> >
> > Then please show a recipe for that.
> 
> Do I need to? The fact that you don't see the problem when executing my 
> scenario means that the above must always work well for you.

I meant a recipe to show one window with the problem and another
without, in the same session.  If you achieve that with the same
resizing of a window, it'd be interesting to see.

> Could you suggest how to proceed debugging it? If you can't reproduce 
> the problem with my scenario, can I send you some data from my session?

The pixel size of the canonical character of that font, as you get in
your session, would be a good start.  E.g., put a breakpoint in
set_cursor_from_row, then step a few lines, and type "pgrow".

Here's the full recipe:

  gdb ./emacs
  break Fredraw_display
  r -Q

Now arrange for the problematic display, and then

  M-x redraw-display RET
  (gdb) break set_cursor_from_row
  (gdb) continue
  (gdb) continue
  (gdb) next
  (gdb) next
  (gdb) next
  (gdb) pgrow

The first "continue" is because the first time set_cursor_from_row is
entered Emacs is displaying the echo area, whereas I need to see the
window of a normal buffer.

> >>> If so, I cannot
> >>> reproduce this: I see both the overlay and the buffer text wrap
> >>> exactly identically.
> 
> Right, same here, albeit (in the first case) when (window-width) returns 
> 34, and (window-pixel-width) returns 288.
> 
> I can also drag the left window border a bit, so that (window-width) 
> still returns 34, but (window-pixel-width) is 291, then the lines in the 
> test buffer become not wrapped.

The fact that pixel size changes, but the size in characters doesn't
is expected, of course.