Re: Merging the underline attribute at EOL

[Eli Zaretskii]

> Date: Thu, 19 Dec 2019 02:19:10 +0100
> From: Ergus <address@hidden>
> Cc: address@hidden
> 
> >>    if (FRAME_WINDOW_P (f)
> >>        && MATRIX_ROW_DISPLAYS_TEXT_P (it->glyph_row)
> >> -      && face->box == FACE_NO_BOX
> >>        && FACE_COLOR_TO_PIXEL (face->background, f) == 
> >> FRAME_BACKGROUND_PIXEL (f)
> >>  #ifdef HAVE_WINDOW_SYSTEM
> >>        && !face->stipple
> >>  #endif
> >>        && !it->glyph_row->reversed_p
> >> -      && !Vdisplay_fill_column_indicator)
> >> +      && !Vdisplay_fill_column_indicator
> >> +      && (NILP (face->lface[LFACE_EXTEND_INDEX])
> >> +          ||(face->box == FACE_NO_BOX
> >> +       && face->underline == FACE_NO_UNDERLINE
> >> +       && face->overline_p == false
> >> +       && face->strike_through_p == false)))
> >>      return;
> >
> >I don't think I understand the test for the :extend attribute being
> >non-nil.  Can you explain why you added it?  In general, faces that we
> >treat as "base face" when we start face merging don't need to have the
> >:extend attribute set (see, for example, the tool-bar face), but we
> >still want to apply them to the space past EOL.  Am I missing
> >something?  Did you bump into some scenario where this attribute
> >caused some regression or bad results?
> >
> >Thanks.
> >
> Hi Eli:
> 
> I just added that test (probably not the best one) for the case where
> the face is the default face. Actually the merge we use (with the
> filter) asserts that the resulting face will be :extend t or the default
> face (AFAIR).

Mostly true (it could also be the "underlying" face when we render a
display string).

> So at this point I thought (probably wrong) that only the default face
> could have :extend nil and we could exit early in that case... But if
> there are some cases that I am not considering we can remove that.

But if the default face has underline or overline or box etc.,
attributes, we don't want to exit early, even though its background
color is the default, right?