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bug#36858: 27.0.50; display bugs with display-fill-column-indicator-mode

From: Ergus
Subject: bug#36858: 27.0.50; display bugs with display-fill-column-indicator-mode
Date: Fri, 9 Aug 2019 00:32:49 +0200
User-agent: NeoMutt/20180716

On Thu, Aug 08, 2019 at 08:31:02PM +0300, Eli Zaretskii wrote:
Date: Wed, 7 Aug 2019 21:46:21 +0200
From: Ergus <address@hidden>
Cc: address@hidden, address@hidden

>In a buffer showing only buffer text (no after-strings at EOB), all
>the glyph rows starting from the one showing EOB have their
>ends_at_zv_p flag set.


In my tests inside extend_face_to_end_of_line the flag ends_at_zv_p is
always false. And for the last line (where it is supposed to be true)
the function extend_face_to_end_of_line is not called at all. So
actually all the code like:

              if (it->glyph_row->ends_at_zv_p)
                it->face_id = default_face->id;
                it->face_id = face->id;

does nothing now.

I see 2 calls to extend_face_to_end_of_line that have an explicit
setting of the ends_at_zv_p flag to 'true' right before the call.  The
fact that you don't see that just means that you are not trying the
use cases where those code fragments are executed.  Which doesn't
surprise me, since the Emacs display engine supports dozens of rare
and subtle use cases, some of which are not easy to even reproduce.

I think that the problem is in the condition:

if (!get_next_display_element (it)) inside display_line that filters the
call to  extend_face_to_end_of_line with:

if (row->reversed_p
    || lookup_basic_face (it->w, it->f, DEFAULT_FACE_ID) !=
      extend_face_to_end_of_line (it);

And needs to be extended probably with with:

|| (!row_text_area_empty (row))

Not sure about which place you are talking: there are several places
in xdisp.c where we call get_next_display_element and test that its
value is zero.

In any case, if we are going to call extend_face_to_end_of_line in
some of the places where we currently don't do that at EOB, we should
condition of the display-fill-column-indicator-mode being ON as well.

Yes I know. I was just trying to find the key condition for our case.

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