Need smooth scrolling to properly trigger animations

I'm testing a page that is a long list of article teasers. There is CSS animation so that as you scroll down the page the articles fade in one by one. In BackstopJS I have a scenario that scrolls to the bottom of the page to trigger the animations. This doesn't actually work as expected because the browsers scrolls instantaneously, so only the first few are triggered.

However, if we alter the engine scripts to use smooth scrolling, everything works as expected.



Start by locating the engine scripts that implement scrolling for BackstopJS scenarios, then reproduce the issue with a long list of animated article teasers and a scenario that scrolls to the bottom. Compare instantaneous and smooth scrolling behavior; done means the scroll reliably triggers the animations before the visual capture.

Written by the indexing model from the issue text.