The top attribute of the background is calculated incorrectly for Messages and other header icons

[gedion]

As you can see in the below image, with clean theme, the background container should be highlighting the messaging icon. But because of the sticky header and scrolling, the offset is incorrectly calculated.

This seems to be happening in the method Tour.prototype.positionBackdrop in usertours/amd/src/tour.js

I've also noticed that the positionBackdrop is being called in Tour.prototype.addStepToPage method before scroll animation is triggered. Calling Tour.prototype.positionBackdrop after the scroll animation seems to fix this issue but I do not know if it is appropriate to do so.