defaultSize for plain spatial window

When I set defaultSize for main plain WindowGroup

WindowGroup {
      ContentView()
    }.defaultSize(width: 500, height: 1000)

I see this bug, my window is clipped a bit at start. Why does it happen?

but when I drag corners a bit it all fixes fine

import RealityKit
import RealityKitContent

struct ContentView: View {
  
  var body: some View {
    
    
    VStack {
      Text("Hello")
      RealityView { content in
        if let scene = try? await Entity(named: "lumoscene", in: realityKitContentBundle) {
          content.add(scene)
          scene.transform.translation = [0.0, 0.0, -0.15]
          for anim in scene.availableAnimations {
            let newAnim = anim.repeat(duration: .infinity)
            scene.playAnimation(newAnim)
          }
        }
      } update: { content in
      } placeholder: {
        ProgressView()
      }
    }
  }
}

#Preview {
  ContentView()
}