[Amp Story] [Page attachments] [Inline] Attachment header doesn't stick to top of viewport

### Description

The header does not stick to the top of the viewport.

Tested in iOS Chrome, & iOS Safari.

https://user-images.githubusercontent.com/3860311/119391948-0aae1400-bc9d-11eb-9f18-9e33c6cb352c.mov

### Reproduction Steps

1. Navigate to [this demo](https://000623720.deployed.codepen.website/) in iOS chrome
2. Open the CTA BUTTONS V2 story
3. Navigate to page 25
4. Swipe up or click the "About The Artist" CTA
5. Drag attachment up past top of viewport.

Header should stick to top of viewport, instead it is scrolling beyond.

### Relevant Logs

_No response_

### Browser(s) Affected

iOS Chrome
iOS Safari

### AMP Version Affected

2105072136000 with experiment `amp-story-page-attachment-ui-v2` forced on in demo story.