It's useful to separate mantra and intent, here. Yes, separation of concerns is a useful thing, but if you just take it to mean that no logic should ever be involved in your templates, you're taking it too far and unnecessarily complicating things. If a section of the page is scrollable or expandable, the scroll and expansion state (and event-handlers) clearly live in the presentation layer and don't belong in any of the data model. (They aren't the only examples, but they are ones that most people accept.)