Buttons on a fragment are not focusable

Question

I have tried every possible way that I can think of to make buttons on a fragment selectable with a D-pad to no avail.

The buttons on a fragment in the layout:

   <RelativeLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content">
        <ImageButton
            android:id="@+id/imageButtonCopyAll"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:src="@drawable/ic_action_copy"
            android:layout_alignParentTop="true"
            android:focusedByDefault="true"
            android:focusable="true"
            android:focusableInTouchMode="true"
            android:contentDescription="Copy all"
            android:onClick="@{(v)->textForEmailWindowFragment.onClick(v)}"
            android:nextFocusRight="@id/imageButtonSave"
            />
        <ImageButton
            android:id="@+id/imageButtonSave"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_toEndOf="@+id/imageButtonCopyAll"
            android:contentDescription="@string/save"
            android:focusable="true"
            android:focusableInTouchMode="true"
            android:onClick="@{(v)->textForEmailWindowFragment.onClick(v)}"
            app:srcCompat="@drawable/ic_action_save" />
    </RelativeLayout>

Java code:

    FragmentManager fragmentManager = getSupportFragmentManager();
    FragmentTransaction fragmentTransaction = fragmentManager.beginTransaction();
    TextForEmailWindowFragment tfewFragment = TextForEmailWindowFragment.newInstance();
    fragmentTransaction.add(R.id.relativeLayoutMain, tfewFragment);
    fragmentTransaction.addToBackStack("foo");
    fragmentTransaction.commit();

Everything works flawlessly with a touchscreen or a mouse. The buttons can be tapped or clicked. However, the buttons never get the focus, so a D-pad cannot select them. The focus is always on the elements of the calling activity's layout beneath the fragment. Could anyone offer a tip on the cause?