REGRESSION: QOpenGLWidget ignore alpha channel as background

If you try to use a QOpenGLWidget as a transparent overlay window, it displays a solid black background instead of transparency in versions after 6.8.3. 

I provide  .cpp/.h file to test with different version, here are my results:

• 6.8.0: works as expected;
• 6.8.3: works as expected;
• 6.9.0: background is completely black; the alpha value of 0.0f is ignored;
• 6.10.0: background is completely black; the alpha value of 0.0f is ignored.

I create this inside main application window with this code:

OpenGlWin* gl = new OpenGlWin();