bug#37774: 27.0.50; new :extend attribute broke visuals of all themes and other packages

[Eli Zaretskii]

> From: Michael Albinus <michael.albinus@gmx.de>
> Cc: Ergus <spacibba@aol.com>,  andreyorst@gmail.com,  37774@debbugs.gnu.org
> Date: Thu, 17 Oct 2019 13:36:26 +0200
> 
> I believe it is more complex. There might be situations foo shall not
> extend to EOL, and other situations it should.
> 
> The latter case is when foo is used in a "rectangular" context as
> explained.

What is a "rectangular context"?  I'm not sure I understand the exact
meaning of that.