How do I get the frame of visible content from SKCropNode?

Question

It appears that, in SpriteKit, when I use a mask in a SKCropNode to hide some content, it fails to change the frame calculated by calculateAccumulatedFrame. I'm wondering if there's any way to calculate the visible frame.

A quick example:

import SpriteKit

let par = SKCropNode()

let bigShape = SKShapeNode(rect: CGRect(x: 0, y: 0, width: 100, height: 100))
bigShape.fillColor = UIColor.redColor()
bigShape.strokeColor = UIColor.clearColor()
par.addChild(bigShape)

let smallShape = SKShapeNode(rect: CGRect(x: 0, y: 0, width: 20, height: 20))
smallShape.fillColor = UIColor.greenColor()
smallShape.strokeColor = UIColor.clearColor()
par.maskNode = smallShape

par.calculateAccumulatedFrame() // returns (x=0, y=0, width=100, height=100)

I expected par.calculateAccumulatedFrame() to return (x=0, y=0, width=20, height=20) based on the crop node mask.

I thought maybe I could code the function myself as an extension that basically reimplements calculateAccumulatedFrame with support for checking for SKCropNodes and their masks, but it occurred to me that I would need to consider the alpha of that mask to determine if there's actual content that grows the frame. Sounds difficult.

Is there an easy way to calculate this?