bug#42347: Feature request: Visual block attribute for overlays

[Eli Zaretskii <eliz@gnu.org>]

> Date: Tue, 14 Jul 2020 07:38:38 +0000
> From: Gregory Heytings via "Bug reports for GNU Emacs,
>  the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>
> 
> >> 2. remove all pixel columns on the right *and on the left* of the 
> >> overlay which have no "content" (that is, no characters on the right, 
> >> and whitespace characters on the left).
> >
> > What would be the definition of "whitespace" for this purpose?
> >
> 
> I think the regexp "[ \t\r\n]*" would be appropriate.

Are you sure?

First, \r doesn't happen in Emacs buffers, and \n is the newline, so
it isn't on the same line.

Next, what about \f, and more generally about any other sequence of
characters that match [:blank:]?

And finally, what about stretches of whitespace generated by the
'space' display properties?