bug#26909: 25.1; A face for margins

[Clément Pit-Claudel]

On 12/05/2020 13.21, Eli Zaretskii wrote:
>> Cc: yuri.v.khan@gmail.com, 26909@debbugs.gnu.org
>> From: Clément Pit-Claudel <cpitclaudel@gmail.com>
>> Date: Tue, 12 May 2020 13:06:00 -0400
>>
>>> Something like that, yes.  But you will need to make sure
>>> extend_face_to_end_of_line is called also for empty lines.  And of
>>> course the "silly" changes need to be made less silly. 
>>
>> Where the less silly strategy would be to only run the extend_face code when 
>> the margins face isn't customized, right?
> 
> The other way around, surely?

Indeed.

>>> And you need
>>> to compute the pixel-width of the stretch glyph, since the code you
>>> cited only places a single SPC character there, which is not what you
>>> want if the margin is wider than one column.
>>
>> Hmm, I think the code already does that?
> 
> Does it where?  space_glyph is initiated in init_display_interactive,
> and I don't see its pixel_width changed anywhere.  What do you get in
> the width of that glyph in the margins?
> 
>>>> +    case MARGIN_FACE_ID:          name = Qmargin;                 break;
>>>
>>> If this is going to be an additional basic face, then why do you call
>>> lookup_named_face  and not lookup_basic_face?
>>
>> It's because I have no idea what a basic face is, so I just cribbed from 
>> places that use the fringe face.  It does things like the following, and I 
>> have no idea what those mean:
>>
>>       face_id = NILP (face) ? lookup_named_face (w, f, Qfringe, false)
>>      : lookup_derived_face (w, f, face, FRINGE_FACE_ID, 0);
>>       if (face_id < 0)
>>      face_id = FRINGE_FACE_ID;
> 
> Why not "crib" from a few lines above the code you hacked, where it
> gets at the default face?

Or from below, where it says "face_id = FRINGE_FACE_ID;"… I don't know: I don't 
know what this code does.  I will try to take the time to understand it; the 
original silly patch was just a few lines thrown together as a proof of concept.