Why my MTKview is drawn like if there is a dark overlay on the top of it?

Question

I'm under macOS and I create an MTKView, and I set this MTKView as a subview of the ContentView of my NSWindow.

when I'm inside my MTKView.drawRect method and when I call NSView(Super).getRectsBeingDrawn it returns me nothing! why?

But my main problem is that the MTKView is drawn darker (like there is something darker on the top of it)

I must have something like this instead:

Any idea what can cause this darker effect?