Need smooth scrolling to properly trigger animations

[dalin-]

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.

[dalin-]

Here's the pull request:

#1112

[garris]

Could this change break or impact performance of existing tests?

[dalin-]

Linting issues fixed. Tests now pass.

I was thinking that this change might also help lazy-loaded images: just scroll to the bottom of the page rather than adding custom JS to force your lazy-load library to load all images.