Android AppBarLayout With BottomNavigationView: Hide Toolbar on Scroll With Fixed BottomNavigationView

TLDR

To avoid BottomNavigationView hidden when scrolling, BottomNavigationView must not be within layout with app:layout_behavior="@string/appbar_scrolling_view_behavior".

Setup Activity with AppBarLayout

AndroidManifest.xml

<?xml version="1.0" encoding="utf-8"?>
<manifest ...>
    <application ...>
        <activity
            android:name=".MainActivity"
            android:label="@string/app_name"
            android:theme="@style/AppTheme.NoActionBar">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
    </application>
</manifest>

res/layout/main.xml

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:fitsSystemWindows="true"
    tools:context=".MainActivity">

    <androidx.coordinatorlayout.widget.CoordinatorLayout
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:fitsSystemWindows="true"
        android:layout_above="@+id/bottomNavView"
        >

        <com.google.android.material.appbar.AppBarLayout
            android:id="@+id/appBar"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:theme="@style/AppTheme.AppBarOverlay">

            <androidx.appcompat.widget.Toolbar
                android:id="@+id/toolbar"
                android:layout_width="match_parent"
                android:layout_height="?attr/actionBarSize"
                android:background="?attr/colorPrimary"
                app:layout_scrollFlags="scroll|enterAlways"
                app:popupTheme="@style/AppTheme.PopupOverlay" />

        </com.google.android.material.appbar.AppBarLayout>

        <!-- <include layout="@layout/main_content" /> -->
        <FrameLayout
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            app:layout_behavior="@string/appbar_scrolling_view_behavior">

            <fragment
                android:id="@+id/nav_host_fragment"
                android:name="androidx.navigation.fragment.NavHostFragment"
                android:layout_width="match_parent"
                android:layout_height="match_parent"
                app:defaultNavHost="true"
                app:navGraph="@navigation/mobile_navigation" />
        </FrameLayout>

    </androidx.coordinatorlayout.widget.CoordinatorLayout>

    <com.google.android.material.bottomnavigation.BottomNavigationView
        android:id="@+id/bottomNavView"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_alignParentBottom="true"
        android:background="?android:attr/windowBackground"
        app:menu="@menu/bottom_nav_menu" />

</RelativeLayout>

NOTE: BottomNavigationView must not be within layout with app:layout_behavior="@string/appbar_scrolling_view_behavior" to avoid being hidden/covered during scroll event.

NOTE: Add app:layout_scrollFlags="scroll|enterAlways" to Toolbar to enable hide toolbar on scroll.

NOTE: Refer Android Prevent BottomNavigationView Cover/Overlap Content/RecyclerView