Florian Müllner authored Nov 13, 2018 and Florian Müllner committed Jul 10, 2019

Window previews are sometimes shown translucent, for example during
drags or animations. They can also have attached dialogs, in which
case the opacity should affect the combination of all windows instead
of being applied to each window individually, blended together, so
make sure they are redirected as a whole when necessary.

#774