Forms Shell bouncy shadow

[justinspencer]

The header and shadow are disconnected when content is scrolled up and down making the component look broken. I've attached a screen recording to simulate the issue.

Bouncy.shadow.mov

As discussed in our meeting, please apply a fixed position to the header shadow so that it doesn't bounce when the content is scrolled up and down. The content bouncing is good and (if possible) I'd like to maintain that behaviour however the shadow must always remain fixed. Please also replicate this behaviour in the progress rope and GEL docs so that everything is consistent.