I2S: layoutbox-invalidate-on-scroll experiment

[jridgewell]

Summary

This accomplishes the main goal of Layers (supporting nested scrolling elements) by invalidating the layout boxes of elements inside a nested scrolling element after a scroll.

Eg, if an <amp-carousel> (once it migrates off of Ownership system) were to scroll, all <amp-img>s inside it would invalidate their layout boxes. Once the next Resources pass happens, the images would be remeasured, and we'll now detect that the image has moved closer/further from viewport.

The layout boxes themselves are still page-absolute and not viewport-relative. This means we keep the speed benefits that stopped us from shipping Layers.

Intent-to-implement (I2I) issue

#25172

Experiment(s) to enable

• layoutbox-invalidate-on-scroll

Required release version

1911042323000, #25319

Demo instructions

Load https://gaitz.github.io/AMP-issues/amp-img-load.html, scroll the carousels

Additional context

/cc @ampproject/wg-approvers

[cramforce]

LGTM

[dreamofabear]

Launched in #26430.