Floating Action Button in an Activity - how to anchor to a recyclerview in a fragment?

Have you tried anchor it to the viewpager?

<android.support.design.widget.FloatingActionButton
    android:id="@+id/fab"
    app:layout_anchor="@id/viewpager"
    app:layout_anchorGravity="bottom|right|end"
</android.support.design.widget.CoordinatorLayout>

Then you extend a FloatingActionButton.Behavior to repond to CoordinatorLayout events:

public class ScrollAwareFABBehavior extends FloatingActionButton.Behavior {

    public ScrollAwareFABBehavior(Context context, AttributeSet attrs) {
         super();
    }

    @Override
    public boolean onStartNestedScroll(CoordinatorLayout coordinatorLayout,
                                   FloatingActionButton child, View directTargetChild, View target, int nestedScrollAxes) {
    return nestedScrollAxes == ViewCompat.SCROLL_AXIS_VERTICAL ||
            super.onStartNestedScroll(coordinatorLayout, child, directTargetChild, target,
                    nestedScrollAxes);
    }

    @Override
    public void onNestedScroll(CoordinatorLayout coordinatorLayout, FloatingActionButton child,
                           View target, int dxConsumed, int dyConsumed, int dxUnconsumed, int dyUnconsumed) {
    super.onNestedScroll(coordinatorLayout, child, target, dxConsumed, dyConsumed, dxUnconsumed,
            dyUnconsumed);

        if (dyConsumed > 0 && child.getVisibility() == View.VISIBLE) {
            child.hide();
         } else if (dyConsumed < 0 && child.getVisibility() != View.VISIBLE) {
             child.show();
         }
     }

 }

And add this to your FloatingButton:

app:layout_behavior="com.package.yourapp.ScrollAwareFABBehavior"

It's well explained here:

https://guides.codepath.com/android/Floating-Action-Buttons

Complementing @petrusgome's answer:

The provided solution didn't work for me. I had an issue where the FAB would not show again after scrolling the recyclerview list down. To fix it, I just had to replace child.hide() with the following code:

child.hide(new FloatingActionButton.OnVisibilityChangedListener() {
    @Override
    public void onHidden(FloatingActionButton fab) {
        super.onHidden(fab);
        fab.setVisibility(View.INVISIBLE);
    }
});

The full explanation can be found here: scrolling issue