Setting color to window dynamically does not give desired output

Hi

I am facing this issue where if window color is set to opaque during initialization and the on some button click, change the color to transparent, it shows black color. Attached a sample reproducible program. Once app is running, there will be a small black colored rectangle. On clicking that rectangle the background color of the window changes.

Issue is, during initialization, if the color is transparent, on clicking rectangle, we see colors changing from transparent to green and vice-a-versa. But if the color is non-transparent during the time of initialization, then we see black instead of transparent.

Let me know if you guys need any further information.