Collapsing parallax toolbar

Five siblings in one Box. Four read the same ScrollState and none of them owns it; the fifth reads no scroll state at all, riding a boolean the toolbar reports back up:

val scroll: ScrollState = rememberScrollState(0)
Box(modifier) {
    Header(scroll, headerHeightPx, imageUrl, backgroundColor, /* … */)   // artwork, parallax + fade
    Body(scroll, headerHeight) { content(color) }                        // the only scrollable
    Toolbar(scroll, headerHeightPx, toolbarHeightPx, onShow = { … })     // pinned bar, appears late
    Title(scroll, title, headerHeight, toolbarHeight)                    // travels between the two
    AnimatedVisibility(showBackButton) { /* floating back button */ }
}

Only Body scrolls, and it reserves the header's space with a spacer rather than containing it: