FixedLayer: Transfer elements on [hidden] toggle

[dreamofabear]

Problem: Fixed elements that are initially hidden may have incorrect DOM ordering WRT fixed elements that are not initially hidden.

Proposed solution: Use MutationObserver to listen for tracked, fixed elements that are un-hidden dynamically by removing hidden attribute (e.g. via amp-bind or toggle/show actions). Transfer those elements to the fixed layer when they're un-hidden.

Context: b/111906735

[jridgewell]

This'll be a 4 phase project:

1. Lint style mutations.
   • Everything currently goes through setStyle/setStyles/setImportantStyles/resetStyles
2. Disallow display style mutations
   • Everything must use toggle helper.
3. Convert toggle helper to use HTLM5 hidden Element attribute.
   • It does the same thing as display style mutation.
4. Add Mutation Observer that listens for hidden Element attribute changes.
   • Update fixed-layer on mutations.

[dreamofabear]

@jridgewell We'll probably need to special-case attributes for amp-access and amp-subscriptions, e.g. amp-access-hide, subscriptions-section, and subscriptions-action.