But I found something interesting, when the mana void spawned and I was in Kitty form my mana did not drain. This means that potentially resto druids could shift to bear or cat and avoid the entire mana drain when they are healing, as opposed to say saving innervates for that part to have some mana.
I haven't yet tested this on normal/hc modes (will do that tonight!) but I would guess it to work the same, because that's how all mana drains in the games have worked thus far.
Anyway hoping some resto druids find this info useful!
Meanwhile Paladin's Beacon of light still doesn't give stacks on the purple glob de-buff and they can heal through it freely.
Either that or the shift needs to happen before the cast happens, as in it may focus on players to drain at the start of the cast as opposed to at the end. But then would healing gained from having mana = healing lost from not being able to cast for so long? Possibly in normal, but I don't think so in Hc.