スクロールでBottom navigationを隠したく、調べたのでメモ。
ヘルパー的なものを自前で用意する必要があるみたい。

BottomNavigationScrollingBehavior.kt

class BottomNavigationScrollingBehavior<V : View>(context: Context, attrs: AttributeSet) :
        CoordinatorLayout.Behavior<V>(context, attrs) {

    override fun layoutDependsOn(parent: CoordinatorLayout?, child: V, dependency: View?): Boolean {
        if (dependency is Snackbar.SnackbarLayout) {
            updateSnackbar(child, dependency)
        }
        return super.layoutDependsOn(parent, child, dependency)
    }

    override fun onStartNestedScroll(
            coordinatorLayout: CoordinatorLayout, child: V, directTargetChild: View, target: View, axes: Int, type: Int
    ): Boolean {
        return axes == ViewCompat.SCROLL_AXIS_VERTICAL
    }

    override fun onNestedPreScroll(
            coordinatorLayout: CoordinatorLayout, child: V, target: View, dx: Int, dy: Int, consumed: IntArray, type: Int
    ) {
        super.onNestedPreScroll(coordinatorLayout, child, target, dx, dy, consumed, type)
        child.translationY = max(0f, min(child.height.toFloat(), child.translationY + dy))
    }

    private fun updateSnackbar(child: View, snackbarLayout: Snackbar.SnackbarLayout) {
        if (snackbarLayout.layoutParams is CoordinatorLayout.LayoutParams) {
            val params = snackbarLayout.layoutParams as CoordinatorLayout.LayoutParams

            params.anchorId = child.id
            params.anchorGravity = Gravity.TOP
            params.gravity = Gravity.TOP
            snackbarLayout.layoutParams = params
        }
    }
}

layout.xml

    <android.support.design.widget.BottomNavigationView
        android:id="@+id/bottom_navigation"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_gravity="bottom"
        app:itemBackground="@color/bottomNavigationItemBackground"
        app:itemIconTint="@color/bottom_navigation"
        app:itemTextColor="@color/bottom_navigation"
        app:layout_behavior="us.sample.presentation.home.BottomNavigationScrollingBehavior"
        app:menu="@menu/bottom_navigation">

    </android.support.design.widget.BottomNavigationView>

レイアウト側でlayout_behaviorを設定してあげればok

参考

このサンプルは動きがカクカクする
https://stackoverflow.com/questions/44777869/hide-show-bottomnavigationview-on-scroll

こっちは滑らか & FABとかSnackbarにも対応してる
https://android.jlelse.eu/scroll-your-bottom-navigation-view-away-with-10-lines-of-code-346f1ed40e9e
https://gist.github.com/ValCanBuild/4bbcb47576e875a17440cf2c438e030a

Material Design Guideline
https://material.io/design/components/bottom-navigation.html#behavior