Signal-Android/app/src/main/res/drawable/message_bubble_background_received_alone.xml

<?xml version="1.0" encoding="utf-8"?>
<layer-list xmlns:android="http://schemas.android.com/apk/res/android">
    <item
        android:bottom="2px"
        android:top="2px">
        <shape android:shape="rectangle">
            <corners android:radius="@dimen/message_corner_radius" />
            <solid android:color="@color/white" />
        </shape>
    </item>
</layer-list>