What's the pattern to pass state up the component tree? Use case: I have some deep child scrollable component, and I want to display a scroll position in the parent. I guess I should pass the mutable state down to this component and observe it in the parent, correct?

yes, that's one right way to do it. If I'm not mistaken

ScrollableColumn

does exactly what you described.

great, thanks!

You also can pass a lambda that updates it

MyChild(onScroll = { scrollPosition.value = it })

fun MyChild(onScroll: (Int) -> Unit)

oh, right, that's even simpler.