tachiyomi/app/src/main/res/drawable/manga_backdrop_gradient.xml

<?xml version="1.0" encoding="utf-8"?>
<shape xmlns:android="http://schemas.android.com/apk/res/android"
    android:shape="rectangle">

    <gradient
        android:angle="90"
        android:endColor="#00ffffff"
        android:startColor="#ffffffff" />

    <corners android:radius="0dp" />
</shape>