Flickering happens when DropShadow is applied

When applying DropShadow effect to Image like below, flickering occurs all the time.

        Image { id: background; source: "clock.png"; visible: clock.night == false
           layer.enabled: true
           layer.effect: DropShadow {
           transparentBorder: true
           horizontalOffset: 3
           verticalOffset: 3
           }
       }

What is known

• Flickering does not happen in Windows desktop environment
• Flickering can also be seen when pop-up window is faded out by animation

How to reproduce
Please run clocks-qtbug108327.zip with "-platform webgl"
clocks-qtbug108327.zip is based on https://doc.qt.io/qt-5/qtdoc-demos-clocks-content-clock-qml.html with above modification.