I noticed a pretty big change how overlays are handled that apply a
face with a background color.

Reproduce as follows:

- emacs -Q

- Put in the scratch buffer:

(defface blue-background
  '((default :background "blue"))
  "blue background")
(setq testov (make-overlay (point-min) (point-max) nil t t))
(overlay-put testov 'face 'blue-background)

- M-x eval-buffer

In Emacs 26, you would see *everything* with a blue background until
(point-max), meaning the blue background would span the whole width of
the window, including the parts without text.

In current Emacs 27.0.91, you see blue background only applied to the
textual parts of the window. The parts without any text have the normal
background color. I have attached screenshots for both cases.

This is on GNU/Linux with Emacs26/27 both compiled from their latest
branches with GTK+ 3.24.5 and plain './configure' (no cairo).

I'm not sure if this is a bug or a deliberate change, but I couldn't
find anything in NEWS mentioning this.

-David