I think "calm" might refer to the user in that context, as the opposite of "frustrated to the brink of violence". A game UI might be wacky and as twitchy as a

I think "calm" might refer to the user in that context, as the opposite of "frustrated to the brink of violence". A game UI might be wacky and as twitchy as a rodent dosed with recreational stimulants, but the user feels calm, capable of navigating through it and never doubting they can accomplish what they set out to do.