AnimatedContent

val transition = stack
    .last()
    .let { top ->
        updateTransition(
            targetState = BackStackState(
                key = Compatible.findKey(top),
                content = top,
                size = size
            ),
            label = null
        )
    }

transition.AnimatedContent(
    contentKey = BackStackState::key,
    transitionSpec = {
        contentTransform(
            difference = targetState.size - initialState.size
        )
    },
    content = { state ->
        stateKeeper.SavedState(
            key = state.key,
            content = {
                Render(state.content)
            }
        )
    }
)

private fun contentTransform(
    difference: Int
): ContentTransform {
    return when {
        difference >= 0 -> {
            ContentTransform(
                targetContentEnter = slideInVertically(
                    initialOffsetY = { height -> height },
                    animationSpec = SlideSpec
                ),
                initialContentExit = fadeOut(
                    animationSpec = FadeSpec
                ),
                targetContentZIndex = 1f
            )
        }
        else -> {
            ContentTransform(
                targetContentEnter = fadeIn(
                    animationSpec = FadeSpec
                ),
                initialContentExit = slideOutVertically(
                    targetOffsetY = { height -> height },
                    animationSpec = SlideSpec,
                )
            )
        }
    }
}

@Immutable
private data class BackStackState(
    val content: Any,
    val key: String,
    val size: Int
)

content