As you can see this is in my main phase. Opponent has just resolved Rift. I had some untapped elves in play that could have added mana (up to 3 mana could have been floated, before Rift resolved).

However the client didnt see floating mana in response as a legal game action and just resolved Rift. (In paper, it is legal to do so.)

I hate having to hold full control for every game but it comes down to it when you cannot code generically for this interaction.

"opponent draws their own card" is very different than 'return target nonland permanent opponent controls'. (This should be something you can generically code. I have even provided some greater details about this problem in my other feedback; recently.)