Signal-Android/res/drawable/recycler_view_fast_scroller_bubble.xml

<?xml version="1.0" encoding="utf-8"?>
<shape xmlns:android="http://schemas.android.com/apk/res/android" android:shape="rectangle">
    <corners android:topLeftRadius="44dp"
             android:topRightRadius="44dp"
             android:bottomLeftRadius="44dp"
             android:bottomRightRadius="0dp"/>
    <solid android:color="#FF0288D1" />
    <size android:height="88dp" android:width="88dp" />
</shape>