[Problem/Bug]: Transparency doesn't work in C# / Win32 / Monogame

[gaetandezeiraud]

What happened?

Here a reproduction https://github.com/gaetandezeiraud/Monogame-WebView2
I want to use WebView2 over Monogame Windows (DirectX and Win32). The problem is that the transparency doesn't work, I have a "grayish color" and not the blue of monogame for the background (who indicate that the transparency was good).

If I resize, it do weird things. With a "black" rectangle and finally the monogame blue.

Importance

Blocking. My app's basic functions are not working due to this issue.

Runtime Channel

Stable release (WebView2 Runtime)

Runtime Version

No response

SDK Version

No response

Framework

Win32

Operating System

Windows 11

OS Version

No response

Repro steps

Check https://github.com/gaetandezeiraud/Monogame-WebView2

Repros in Edge Browser

No, issue does not reproduce in the corresponding Edge version

Regression

No, this never worked

Last working version (if regression)

No response