bug#38731: [PATCH] Calculate first glyph to be redrawn when exposing (bug#38731)

[Eli Zaretskii <eliz@gnu.org>]

> Date: Fri, 27 Dec 2019 19:06:52 +0000
> From: Alan Third <alan@idiocy.org>
> Cc: netjune@outlook.com, 38731@debbugs.gnu.org
> 
> The actual problem is that if there is a face that’s to extend to the
> end of the line (for example with global-hl-line-mode) then
> expose_area asks draw_glyphs to draw glyph 0 at x coordinate 0, even
> though in this case there is horizontal scrolling in effect that means
> glyph 0 should be at a negative x coordinate.
> 
> In my case I could see that row->x in expose_area was set to -42 but
> expose_area called draw_glyphs with an x parameter of 0. This offset
> the glyphs on the screen by 42 pixels. I could see this both in
> debugging printfs and by actually measuring the difference between the
> cursor and the equivalent text as drawn on the screen.

You are saying that, when the window is hscrolled, the glyphs in the
glyph row start with the glyph for the very first character of the
physical line in this case?  IOW, if the line's text is "abcdef", then
row->glyphs[TEXT_AREA] is the glyph for the character 'a', even though
the line is hscrolled?

This shouldn't happen, because display_line, the function which
produces glyphs in glyph rows, starts producing the glyphs at the
first one that is actually visible, thus taking the hscroll into
account.  See the code in display_line that follows this comment:

  /* Move over display elements that are not visible because we are
     hscrolled.  This may stop at an x-position < first_visible_x
     if the first glyph is partially visible or if we hit a line end.  */
  if (it->current_x < it->first_visible_x + x_incr)

So if in your case you get a glyph row with all the characters,
including those which are scrolled out of the viewport, then something
is wrong where the glyph row is prepared, not where it is displayed.
Specifically, when the window is hscrolled, display_line is called
again to produce the glyphs starting from the first visible glyph.
Perhaps in this case the NS display machinery uses a stale glyph row,
from before the hscrolling?