Here is a sample design (make the preview wide -- ideally 800px). https://jsfiddle.net/Brunni/2n3w33g9/
Basically it's a bunch of divs nested with flex and everything works well until container. Then I have the transitioner div which serves for push-right-to-left transitions (I put two example template-wrappers that are currently supposedly scrolling).
I don't understand why the transitioner div is computed as height=0. I guess children can not have a dependency on the parent since the parent is flowed depending on the children, but I couldn't find a statement that this is absolutely true.
Then things get worse since I need to absolute-position the template-wrappers that will be animated. My ultimate goal is to have the scroller/template-wrapper to be 100% of the container even though they are absolute-positioned. Is that even possible?