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QML DropShadow is an interesting effect that acts in a very simple way. It works fine in my application and produces the following result:

The only disadvantage of DropShadow effect is that is slows down my application from 60 FPS to 30 FPS on Android Phone. The following code demonstrates how I use it with StackView:

import QtQuick 2.7
import QtQuick.Controls 2.1
import QtGraphicalEffects 1.0

ApplicationWindow {
    id: window
    title: appName

    StackView {
        anchors.fill: parent
        id: stack
        initialItem: Item {
            //...
            //further controls goes here
            //...
        }
    }

    DropShadow {
        anchors.fill: stack
        horizontalOffset: 3
        verticalOffset: 3
        radius: 8.0
        samples: 17
        color: "#80000000"
        source: stack
    }
}

So I think it would be better to draw the shadow for all the buttons with Photoshop, for example.