[AppBarLayout] liftOnScroll not detecting scrolling on top of nested horizontal RecyclerView

[BenjyTec]

Description:

I have a CoordinatorLayout with an AppBarLayout and a NestedScrollView. In the NestedScrollView, I have a horizontally scrolling RecyclerView surrounded by two dummy Views.

I have set liftOnScroll="true" and it is working when I scroll on dummy Views in the NestedScrollView, But when I vertically scroll on top of the horizontal RecyclerView, while the NestedScrollView correctly scrolls vertically, the AppBarLayout seems not to detect it, and does not update its lifted state. When I fling on top of the RecyclerView with a lot of velocity, it seems to work correctly most of the time. However it never works correctly when slowly dragging on top of the RecyclerView.

Expected behavior:

The liftOnScroll should update the AppBarLayout when the NestedScrollView is scrolled, no matter where exactly the vertical scroll gesture was started inside of the NestedScrollView.

Source code:

MainActivity.java

public class MainActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        EdgeToEdge.enable(this);
        setContentView(R.layout.activity_main);
        getWindow().setNavigationBarColor(SurfaceColors.SURFACE_2.getColor(this));

        RecyclerView recyclerView = findViewById(R.id.recyclerView);
        LinearLayoutManager mLayoutManager = new LinearLayoutManager(getApplicationContext(), LinearLayoutManager.HORIZONTAL, false);
        recyclerView.setLayoutManager(mLayoutManager);
        recyclerView.setHasFixedSize(false);
        recyclerView.setAdapter(new StatsAdapter());
    }
}

activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<androidx.coordinatorlayout.widget.CoordinatorLayout
    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">

    <com.google.android.material.appbar.AppBarLayout
        android:id="@+id/mainBar"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:fitsSystemWindows="true"
        app:liftOnScroll="true">
        <com.google.android.material.appbar.MaterialToolbar
            android:id="@+id/toolbar"
            android:layout_width="match_parent"
            android:layout_height="?attr/actionBarSize"
            android:gravity="center" />
    </com.google.android.material.appbar.AppBarLayout>

    <androidx.core.widget.NestedScrollView
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:id="@+id/nestedScrollViewStats"
        app:layout_behavior="@string/appbar_scrolling_view_behavior">
        <LinearLayout
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:id="@+id/linearLayoutStats"
            android:orientation="vertical"
            android:layout_gravity="center_horizontal">
            <View
                android:layout_width="match_parent"
                android:layout_height="400dp" />
            <androidx.recyclerview.widget.RecyclerView
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:background="#F44336"
                android:id="@+id/recyclerView" />
            <View
                android:layout_width="match_parent"
                android:layout_height="400dp" />
        </LinearLayout>
    </androidx.core.widget.NestedScrollView>
</androidx.coordinatorlayout.widget.CoordinatorLayout>

StatsAdapter.java

public class StatsAdapter extends RecyclerView.Adapter<StatsAdapter.ViewHolder> {

    public StatsAdapter() {}

    public static class ViewHolder extends RecyclerView.ViewHolder {

        public TextView textViewCalc;

        public ViewHolder(View itemView) {
            super(itemView);
            this.textViewCalc = itemView.findViewById(R.id.textViewCalc);
        }
    }

    @Override
    public StatsAdapter.ViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {
        LayoutInflater layoutInflater = LayoutInflater.from(parent.getContext());
        View listItem = layoutInflater.inflate(R.layout.stats_item, parent, false);
        ViewHolder viewHolder = new ViewHolder(listItem);
        return viewHolder;
    }

    @Override
    public void onBindViewHolder(ViewHolder holder, int position) {
        holder.textViewCalc.setText(
                position + " x " + position + "\n" + position + " x " + position + "\n" + position + " x " + position
        );
    }

    @Override
    public int getItemCount() {
        return 9;
    }
}

stats_item.xml

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:id="@+id/statsItem"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_margin="8dp">

    <TextView
        android:id="@+id/textViewCalc"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:gravity="center"
        tools:text="X x Y"
        android:textSize="16sp"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintStart_toStartOf="parent" />

</androidx.constraintlayout.widget.ConstraintLayout>

Minimal sample app repro:

https://github.com/BenjyTec/TopBarRepro

Android API version:

API35, but also reproduced back until API21.

Material Library version:

Issue reproduced on 1.12.0 and 1.13.0-alpha08

Device:

Google Pixel 4a, and Emulator

Attachments:

liftonscroll.mp4

[imhappi]

@hunterstich do you know why this would be happening?