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

[Eli Zaretskii]

> Date: Wed, 8 Jul 2020 19:19:05 +0200 (CEST)
> From: Gregory Heytings via "Bug reports for GNU Emacs,
>  the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>
> 
> 1. calculate the overlay it it would have been displayed by Emacs 21-26.
> 
> 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?