javascript - How does Ionic fade away page header elements when closing?

In Ionic it's possible to use a left-to-right swipe to close a page, see animated gif. During this swipe, some header elements fade away, based on the position of the swipe. I've really looked into the CSS but can't seem to find any clues how this is done.

I want to use these CSS changes based upon swipe movement for my own development, and using this Ionic example really clarifies what I want.

To test the Ionic example, use Firefox or Safari (Ionic doesn't use this fading in Chrome), and enter responsive/mobile mode.

https://ionic-4-navigation.firebaseapp.com/