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Re: Fill column indicator functionality

From: Clément Pit-Claudel
Subject: Re: Fill column indicator functionality
Date: Fri, 15 Mar 2019 10:35:02 -0400
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.5.1

On 15/03/2019 10.12, Eli Zaretskii wrote:
>> From: Clément Pit-Claudel <address@hidden>
>> This was actually going to be my follow-up question :) I see lines of 
>> variable height in many of most of my Emacs mode, through a combination of 
>> factors:
>> - Font switches due to symbols picked by prettify-symbols-mode not being 
>> available in my main programming font
>> - Variable-pitch fonts being used by AucTeX for section and paragraph titles 
>> - Varying font sizes to indicate nesting depth in Org-mode
>> - Changing line spacing to separate top-level items in org-mode
> Does the fci mode support those use cases?

Not well, no.  There are discontinuities, and if I use wide characters (like 
CJK spaces, e.g. ' '), portions of the line are misaligned.

> But yes, we have several methods of scaling display elements which
> could be used to solve these use cases.  I just don't think we should
> block the feature until they are fixed.  However important are the use
> cases you enumerated, they are not the majority in Emacs, right?

No, I don't think they are the majority of Emacs :) 

>> Also, as a second follow-up question: we already have code to draw boxes 
>> around characters; would it be hard to extend that code to allow 
>> customization of which sides to draw? Alternatively, would it be hard to add 
>> a vertical strike-through face property? (I've used horizontal 
>> strike-through as a way to display horizontal lines in the past)
> I'm not sure where are you going with this.  Yes, Emacs is capable of
> drawing horizontal and vertical lines of specified pixel-width, but
> the issue here is not whether we can do it, the issue is how to do
> that in a way that fits as much as possible the current basic design
> and implementation of the display engine.  Otherwise, the job becomes
> so large that we probably won't have this feature any time soon.
> Specifically, drawing a "vertical strike-through" still doesn't solve
> the problem with the height of the line, because the glyph we will
> "strike through" will still need to have a suitable height, right?

Maybe? At the moment boxes and backgrounds already span the full line height, 
so I'm assuming strike-throughs would as well.
All that I was pointing out is that the box display code already draws lines 
that reliably span the entire line height.


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