[Reg 6.8->6.9] Windows: Semi-transparency broken for frameless windows with OpenGL

During the process of implementing a custom window using Qt.FramelessWindowHint, I found that when using OpenGL as the backend of the RHI, setting a color that includes an alpha channel for the window has no effect. This happens only on windows platform, since Qt6.9.0.

The demo code:

import QtQuick

Window {
    width: 640
    height: 480
    visible: true
    title: qsTr("Hello World")
    flags: Qt.Window | Qt.FramelessWindowHint
    color: "#800000FF"
} 

Create a new "Qt Quick Application" with the QtCreator, replace the code in Main.qml, and run the compiled app with "QSG_RHI_BACKEND=opengl" environment.