Added in https://gerrit.wikimedia.org/r/#/c/341100/1 (8 months ago).
I think we shouldn't have added this feature for two reasons:
- Scroll-linked rendering effects have a detrimental effect on hardware accelerated scrolling, and are specifically called out with a console warning in Firefox: https://developer.mozilla.org/en-US/docs/Mozilla/Performance/Scroll-linked_effects. There may be exceptional cases where this is necessary, but we should avoid and discourage them by default, and try to use other approaches.
- I don't think elements disappearing because they are partially out of the viewport meets user expectations, and in the case where the viewport is of a restricted size may be frustrating.