Demo Direct3D 11 Under QML cannot replace the root Item with Rectangle

Previous Question

Qt Demo Direct3D 11 Under QML

Replacing the root Item of main.qml with Rectangle will cause D3D11Squircle to become white color. When i set the transparent color to the Rectangle,   D3D11Squircle can display normal.

How to deal with this problem?

Current Question

I read the qt document, and find the details of afterRendering(): "... Rather, it is the combination of beforeRendering() + beforeRenderPassRecording() or beforeRendering() + afterRenderPassRecording() that is typically used to achieve under- or overlaying of the custom rendering." So i tried to change the demo's signal "beforeRenderPassRecording" to signal "afterRenderPassRecording",  but the custom rendering is still underlaying.