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

[Drew Adams]

> How this highlighting is different from the rectangle selection mode
> activated by 'C-x SPC'?

I'm not the OP, but I think the point is to provide
different behaviors for overlays, just as we now
have the original behavior (rebaptized as :extend)
and the new behavior that doesn't extend.

It's not about text selection or defining the region.
It's about overlay properties.  That's my understanding,
anyway.