martin rudalics <rudalics@gmx.at> writes:

>>> (require 'auto-dim-other-buffers)
>>> (set-face-attribute 'default nil :background "red")
>>> (set-face-attribute 'auto-dim-other-buffers-face nil :background "blue")
>>> (auto-dim-other-buffers-mode 1)
>>>
>>> It uses the :filtered feature introduced in Emacs 27.
>>
>> Oh, cool.  I guess there isn't more to do here, then...
>
> I doubt it would work when showing the same buffer in two windows.

I downloaded the package to test it now, and somehow it does: