bug#42307: Feature request: Visual block attribute for overlays

[Gregory Heytings via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

>
>> In fact, I'm not entirerly sure that explaining (or implementing) that 
>> behavior with regexpes or character is the best thing to do.  It's a 
>> matter of visual representation.  Perhaps the following explanation is 
>> clearer (or more precise): draw the overlay with :extend t, and remove 
>> pixel columns on the left and on the right that are displayed in the 
>> same way as a whitespace character would have been displayed.
>
> Well, it's not that simple.  For starters, there's no "pixel columns" in 
> the display engine, at least not on GUI frames.
>

That's what I thought indeed.  My explanation was a specification, not an 
implementation hint ;-)

>
> More to the point, I think the feature you requested will be not very 
> simple to implement, because it requires the display code to do 
> look-ahead: when it lays out leading whitespace, it should consider 
> characters after that.  That calls for some complications, because the 
> display code basically inspects characters one by one and makes (almost) 
> all the decisions on the fly.
>
> So someone will have fun implementing this.  Patches welcome.
>

I'm not sure I have the necessary background to do this (it's a 
euphemism), but I'll try to see what I can do.

Gregory