Transparent titlebar under macOS 10.10+

When enabling NSWindowStyleMaskFullSizeContentView on the main window, the content view is black. Seems like a rendering issue: if you minimize the app in the dock and show it again, the content is now rendered but turn back to black if you resize the window. And same if you switch desktop, or anything that hide then put back the window on screen.

It also flickers with the rendered content when you start a resize by dragging the borders.

What I would like to have is a transparent titlebar with the traffic lights, like some Apple apps.

I joined a minimal project that reproduce the issue. I didn't find any solution to fix it, obviously  Maybe someone can leave some hints, or a solution!

Thanks,

Benjamin