How to Create Transparent extra-viewport?

[JamboChen]

I'm developing a game utility application using egui, where the main viewport is transparent and always on top. This has been working perfectly when the viewport is the main one, started with eframe::run_native and impl the eframe::App trait with clear_color and update functions.

However, I've encountered an issue when trying to create a transparent extra-viewport using show_viewport_deferred. While the extra-viewport retains its style, it is no longer transparent. I suspect this is because the clear_color function is no longer called for the extra-viewport. The temporary main viewport, which also impl clear_color function, becomes transparent instead.

Is there any way to make the transparent extra-viewport drawn with show_viewport_deferred ?