Any thought to this just being an optional method on WidgetBaseInterface?

also, you will essentially always do a noop on most widgets on attachment, and I don't know the performance impact, but invoking potentially hundreds of the same function seems 😕 where as testing for presence and only calling if present feels like it would be more efficient...

We do the same for the onElementCreated / onElementUpdated, these guys get called on requestIdleCallback so certainly should reduce/negate the performance impact - but I can look at the optional method.

Problem is I don't want to put it on the interface (it's protected), so I don't think we can type it as such.

Yeah, optional abstract isn't allowed and protected cannot be enforced on an interface... You ruin all my best ideas... 😝