✨Handling sticky, fixed and hidden elements in amp-next-page v2

[wassgha]

Changes

• Breaking: In contrast to amp-next-page v1 fixed elements will no longer be hidden by default, they will however be constrained to their document's bounds (they won't overflow beyond the document's borders when the document is not visible).
• Introduces two new global attributes amp-next-page-hide, and amp-next-page-replace:

Attribute	Value	Description
amp-next-page-hide	None	Can be added to any element that should only appear in the host document (and not in subsequently loaded pages)
amp-next-page-replace	Unique ID	Denotes elements that are shared between the host document and subsequent next pages. The element belonging to the last visible page will replace any previous instances of the same ID

Demo page: Demo Site

[amp-owners-bot]

Hey @ampproject/wg-caching, these files were changed:

• validator/validator-main.protoascii

[Gregable]

Validator changes look good, especially since they are just a comment change.

[caroqliu]

Nice job! Don't forget to add unit tests and update the documentation file with the new attributes.

[caroqliu]

This looks a lot more readable and clear now, thanks!