WindowContainer cannot handle transparent Window properly

Reproducer attached. Problem has been confirmed in 6.8.3, 6.9.0 and 6.10.0 "snapshot".

Build and run the reproducer, and the window looks OK upon being shown. That a transparent Window embedded in a WindowContainer is on top of a green background Window. And you can also see a purple Rectangle within transparent Window area. So this is how I expect it to look like:

The problem is whenever you resize the main window. The transparent Window is now black:

Another issue is that you have to have "opacity: 0.999" (or some value very close to 1) for the transparent Window. I cannot use "opacity: 0" to replace "color: "transparent"" since that is going to make everything within transparent Window invisible. I cannot us the default value of 1 either since that makes transparent Window opaque white, like:

So to make the contents in transparent window look as normal as possible but still keep window itself transparent, "opacity: 0.999" is an awkward workaround.

Also notice that I have specified "flags: Qt.FramelessWindowHint" to transparent window. It does not help though.