The dark area behind the AlertDialog does not fill the entire screen

Question

I am using AVD for the tablet Pixel C with API 15. When a system or application displays an alert dialog, the dark area behind the dialog does not fill the entire screen, like:

All handsets work normally. Tablet AVD for API 19 and late work fine too.

For activity layout I used coordinatorlayout, like:

<androidx.coordinatorlayout.widget.CoordinatorLayout 
    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"
    android:fitsSystemWindows="true"
    android:background="@color/background_content"
    tools:context=".MainActivity">
    ...
</androidx.coordinatorlayout.widget.CoordinatorLayout>

Question: What is the reason for such an incorrect display: in my application, emulator, or API level? Will this be displayed normally on the physical device?

Thanks in advance for any answer/comment!