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

[Eli Zaretskii <eliz@gnu.org>]

> Date: Tue, 14 Jul 2020 17:01:43 +0000
> From: 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.

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.