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

<?xml version="1.0" encoding="utf-8"?>
<selector xmlns:android="http://schemas.android.com/apk/res/android">
    <item android:state_pressed="true">
        <layer-list>
            <item android:drawable="@drawable/icon_button_squircle" />
            <item>
                <shape android:shape="rectangle">
                    <solid android:color="@color/signal_colorSecondaryContainer_12" />
                    <corners android:radius="18dp" />
                </shape>
            </item>
        </layer-list>
    </item>
    <item android:drawable="@drawable/icon_button_squircle" />
</selector>