AdditionalStylingPart on widgets outside of Flows (OSOE-595)

[sarahelsaig]

I've noticed that AdditionalStylingPart gets welded to widget content items even when you add them to a zone durectly. Yet there is no visible editor for it and I don't think it actually does anything either. So you only see it when you export a content recipe, but when that happens I think this is a bit confusing.

We could do two different things, I'm not sure which would be more correct:

• Somehow detect if the content item is inside a flow and welding AdditionalStylingPart if it's not. (Currently AdditionalStylingPartHandler only checks if the content it's definition has the "Widget" stereotype.)
• Make the AdditionalStylingPart effective on regular zone-bound widgets too and display a simple editor. For example apply the CustomClasses to the widget-container.

Jira issue

[0liver]

The second option sound desirable!