android - Inconsistent shadow on MaterialCardView when its overlaid by children

Question

When a child of a MaterialCardView fully overlays it, the shadow under it disappears. I have tried setting elevation to the view itself but to no effect.

Example layout:

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">

    <!-- Elevation of this card is large for demonstration purposes -->
    <com.google.android.material.card.MaterialCardView
        android:layout_width="match_parent"
        android:layout_height="90dp"
        android:layout_margin="16dp"
        app:cardElevation="24dp"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent">

        <View
            android:layout_width="120dp"
            android:layout_height="match_parent"
            android:background="@color/purple_500"
            android:elevation="24dp"
            app:elevation="24dp" />
    </com.google.android.material.card.MaterialCardView>

</androidx.constraintlayout.widget.ConstraintLayout>

Screenshot of the problem:

Here shadow is clearly visible under white background of the card, but not under purple view.

The app was tested on API 30 emulator, but I have observed similar behaviour on API 29 physical device.

question from:https://stackoverflow.com/questions/65642209/inconsistent-shadow-on-materialcardview-when-its-overlaid-by-children

Answer 1

Ok it turns out that it was just an optical illusion: if you overlay the purple control in Paint the shadow is consistent:

Now its up to me how to prevent the following illusion from impacting the visual experience of my app.