The reason Istredd is spending so much time in this room has a lot to do with how many wards and protections there are in place. It wouldn't be obvious to anyone without magical ability, but anyone trying to make trouble in there would very quickly regret it. This is the safest place in the castle for him. He is trying to do better about it.

So he does not immediately jump at the knock since it is safe in there, and it's probably someone who knows him checking in. That is proven right when Natasha's head comes into view and he smiles, setting aside the journal and his notes. He is sitting cross-legged on his bed, a common pose especially since it's how he goes into the Horizon.

He gestures and the door open for her so she doesn't have to worry about balancing the tea. Getting back his Chaos has been important and he maybe is overusing it for things. "Please come in, Natasha."