bug#14575: 24.3.50.1; Overlays with a window property affect other windows

[Dmitry Gutov]

Eli Zaretskii <eliz@gnu.org> writes:
> Btw, this bug was introduced 2 years ago, during the development of
> v24.1, and no one ever said anything until now.  I guess that speaks
> volumes of how frequently people use window-specific overlays.

company-mode does use them, but with `before-string' and `invisible'
instead of `display'. Not exactly sure why.