Coordinate nested scrolling

Consume an ancestor effect

Attach one stable NestedScrollConnection to the ancestor that owns the effect. Pre callbacks run before child consumption; post callbacks receive what the child consumed and what remains. Return only the signed distance or velocity actually consumed.

fun UiTreeBuilder.CollapsingToolbar(
    collapseBy: (deltaY: Float) -> Float,
) {
    val latestCollapseBy = rememberUpdatedState(collapseBy)
    val connection = remember {
        object : NestedScrollConnection {
            override fun onPreScroll(
                available: ScrollDelta,
                source: NestedScrollSource,
            ): ScrollDelta {
                return ScrollDelta(
                    x = 0f,
                    y = latestCollapseBy.value(available.y),
                )
            }
        }
    }

    Column(modifier = Modifier.nestedScroll(connection)) {
        Text("Collapsing toolbar")
        ScrollableColumn {
            repeat(40) { index -> Text("Row $index") }
        }
    }
}

Clamp application state inside collapseBy; the Android renderer also rejects non-finite or over-consumed results. Use NestedScrollSource when user input, fling continuation, and imperative side effects require different policy.

Dispatch a custom scroll source

Pass a stable NestedScrollDispatcher to nestedScroll when custom gesture or programmatic code must enter the same parent chain. Dispatch pre-scroll first, consume the remainder locally, then dispatch post-scroll with the local consumption and remainder. Use the equivalent pre/post fling pair for velocity.

Lazy collections, pagers, eager scroll containers, PullToRefresh, and framework drag or transform pan participate in the same chain. A native AndroidView joins automatically only when its View implements Android nested scrolling; otherwise dispatch explicitly. See Modifier architecture for phase order, modifier ordering, AndroidX mapping, and the legacy native-fling limitation.