Prevent PKCanvasView background from turning white while drawing

Question

I have a PKCanvasView in my SwiftUI app using a UIViewRepresentable.

I want the canvas content (PKDrawing) to have a white background and the rest of the view a gray background. The canvas is zoomable.

Here is the relevant bits of my code:

import SwiftUI
import PencilKit

struct ContentView: View {
    
    var body: some View {
        GeometryReader { geometry in
            ZStack {
                Color(.systemGroupedBackground)
                CanvasView()
            }
        }
    }
}

struct CanvasView: UIViewRepresentable {
    
    func makeUIView(context: UIViewRepresentableContext<CanvasView>) -> PKCanvasView {
        
        let canvasView = PKCanvasView()
        canvasView.drawingPolicy = .anyInput
        canvasView.contentSize = CGSize(width: 1000, height: 1000)
        
//        canvasView.backgroundColor = .clear
//        canvasView.isOpaque = false

        canvasView.minimumZoomScale = 0.5
        canvasView.maximumZoomScale = 4

        return canvasView
    }
    
    func updateUIView(_ view: PKCanvasView, context: UIViewRepresentableContext<CanvasView>) {
        view.setZoomScale(0.5, animated: true)
    }
    
    func makeCoordinator() -> CanvasView.Coordinator {
        return CanvasView.Coordinator(self)
    }
    
    class Coordinator: NSObject, PKCanvasViewDelegate, PKToolPickerObserver {
        
        var parent: CanvasView
        
        init(_ canvasView: CanvasView) {
            self.parent = canvasView
        }
    }
}

Initially this looks fine and the drawing has a white background as expected with a gray background beyond the bounds of the drawing. However while drawing a stroke, the entire screen background becomes white including the gray parts outside the bounds of the drawing. It reverts back to normal as soon as the stroke is complete.

I have tried to set canvasView background to clear and isOpaque to false but then there is no longer any white background for the drawing which I want to keep.

I imagine having the drawing have a background different from the screen background is common so I wonder what I am doing wrong here.