How to make AppBarLayout background transparent

The same problem happened to me and at this point it has nothing to do with the transparency of the AppBarLayout. But the AppBarLayout pushes the content of your NestedScrollView below itself when fading in.

You can solve this by

1. moving your NestedScrollView up and behind the AppBarLayout and
2. adding a space element to your NestedScrollView with the size of the AppBar

Step 1 can be easily achieved by adding this to your onCreate()

mNestedScroll.getViewTreeObserver().addOnGlobalLayoutListener(new ViewTreeObserver.OnGlobalLayoutListener() {
    @Override
    public void onGlobalLayout() {
        mNestedScroll.getViewTreeObserver().removeOnGlobalLayoutListener(this);

        final int appBarHeight = mAppBar.getHeight();
        mNestedScroll.setTranslationY(-appBarHeight);
        mNestedScroll.getLayoutParams().height = mNestedScroll.getHeight() + appBarHeight;
    }
});

For Step 2 you need to add an empty view with the height of the appBar as a spacer to your NestedScrollView. Note: I used a RecyclerView instead of a NestedScrollView in my approach, so I had to add an empty ViewHolder with the appBarHeight to my RecyclerView. So this snippet is not tested, but it should do the trick for you:

<View
    android:layout_width="match_parent"
    android:layout_height="?attr/actionBarSize"/>

Another option is to add negative top margin to NestedScrollView (or whatever view is using appbar_scrolling_view_behavior) and the same positive top margin (or padding) to its direct child:

<androidx.core.widget.NestedScrollView
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    app:layout_behavior="@string/appbar_scrolling_view_behavior"
    android:layout_marginTop="-100dp" >

    <androidx.constraintlayout.widget.ConstraintLayout
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:paddingTop="100dp" >

        ...

    </androidx.constraintlayout.widget.ConstraintLayout>

</androidx.core.widget.NestedScrollView>

<com.google.android.material.appbar.AppBarLayout
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:background="@null" >

    <com.google.android.material.appbar.CollapsingToolbarLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        app:layout_scrollFlags="scroll|exitUntilCollapsed" >

        ...

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

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

Note that in order for this approach to work the AppBarLayout should be placed below NestedScrollView in the layout-file. In case fading edges of NestedScrollView produce visual artifacts, add android:overScrollMode="never" to its parameters. Also take care when using elevation since it, when used in wrong place (say, when applied to whole AppBarLayout), may also produce visual artifacts.