Block Multi-Window Use
Marshmallow 6.0.1 does not support multi-window. It is hidden in the official releases. There are hacks to enable it by rooting the device and change some system setting files. But that's not supported by Funamo. In general, we do not recommend that the device be rooted because it gives user authorities to change system.
That said, Funamo app blocking works for multi-window case. Android's next release N (currently in preview stage) will officially support multi-window and we have tested it. The app blocking works fine for Android N.
I am checking with our engineering team to see if there is any work around for this unofficial feature of Marshmallow. I will keep you posted. I am creating a separate answer just to be clear that this is simply a workaround (if any), not an officially supported feature.