Popup window with recycler view background not working

To get your desired look you can modify your popup view xml like this:

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="wrap_content">

    <androidx.cardview.widget.CardView xmlns:app="http://schemas.android.com/apk/res-auto"
        android:id="@+id/cardView"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_marginTop="10dp"
        android:layout_marginStart="10dp"
        android:layout_marginEnd="20dp"
        android:layout_marginBottom="10dp"
        app:cardCornerRadius="6dp"
        app:cardElevation="6dp">

        <androidx.recyclerview.widget.RecyclerView
            android:id="@+id/rvCategory"
            android:layout_width="match_parent"
            android:layout_height="200dp"
            android:paddingStart="10dp"
            android:paddingTop="5dp"
            android:paddingEnd="1dp"
            android:paddingBottom="5dp"
            android:clipToPadding="false"
            android:scrollbars="vertical" />
    </androidx.cardview.widget.CardView>
</LinearLayout>

and in the code where you initialize your popup it should be like this(here mContext is your Activity):

PopupWindow mPopup = new PopupWindow(mContext);
View popUpView = mContext.getLayoutInflater().inflate(R.layout.your_popup_xml, null);
// other initializations
mPopup.setBackgroundDrawable(null);
mPopup.setContentView(popUpView);
mPopup.setFocusable(true);
mPopup.setClippingEnabled(true);