Before building out T197718 we want to determine how to instrument and test this feature.
@ovasileva @Tbayer @alexhollender @phuedx @Jdlrobson met to discuss options. We'll update this card with a summary of that conversation and associated spikes we'll need to carry out to do so.
 scroll event is triggered by user interactions, internal links (e.g. #Heading) and opening overlays (notably clicking images opens the media viewer overlay which triggers a scroll to top). @phuedx pointed out that looking at `event.currentTarget.srcElement` will allow us to distinguish scrollbar-triggered (`srcElement === document`) and UI element triggered scrolls.
 We need to decide whether to send multiple events per page or follow an approach similar to https://meta.wikimedia.org/wiki/Schema:MobileWikiAppPageScroll
 We'll need to determine whether an A/B test is needed before building out T197718. In particular that decision will steer how engineers implement this.