car-ui-lib/res/layout/car_ui_preference_dropdown.xml

<?xml version="1.0" encoding="utf-8"?>
<!--
    Copyright 2019 The Android Open Source Project

    Licensed under the Apache License, Version 2.0 (the "License");
    you may not use this file except in compliance with the License.
    You may obtain a copy of the License at

         http://www.apache.org/licenses/LICENSE-2.0

    Unless required by applicable law or agreed to in writing, software
    distributed under the License is distributed on an "AS IS" BASIS,
    WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    See the License for the specific language governing permissions and
    limitations under the License.
-->

<FrameLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="wrap_content">
    <!-- This spinner should be invisible in the layout and take up no space, when the Preference
         is clicked the dropdown will appear from this location on screen. -->
    <Spinner
        android:id="@+id/spinner"
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        android:layout_marginStart="@dimen/car_ui_preference_dropdown_padding_start"
        android:visibility="invisible"/>

    <include layout="@layout/car_ui_preference"/>

</FrameLayout>