How to avoid the traffic light buttons on iPad

You can't eliminate the traffic light but you can specify its style a bit.

In UIKit you can implement the UIWindowSceneDelegate method preferredWindowingControlStyle(for:) and return one of three styles.

I don't know the SwiftUI equivalent.

I filed FB18559686 for this, and was just discussing the same issue over in https://developer.apple.com/forums/thread/790994

I posted a sort-of solution in https://developer.apple.com/forums/thread/790994 that shows a mostly working way to detect the frame of the window controls

I found that I could use the containerCornerInsets on the GeometryProxy to get this frame. (https://developer.apple.com/documentation/swiftui/geometryproxy/containercornerinsets)

Here is my code where I added a red rectangle behind the controls.

GeometryReader { geo in
    NavigationSplitView(columnVisibility: $sceneModel.columnVisibility) {
       
        GPSidebarView(appModels: self.appModels())
            .sceneWidth(geo.size.width)
        
    } detail: {
        ZStack(alignment: .topLeading) {
            GPRootContentViewIOS26(appModels: self.appModels())
                .cardStackVisible(sceneModel.hasCardOnMainStack)
            
            Rectangle()
                .fill(.red)
                .frame(width: geo.containerCornerInsets.topLeading.width, height: geo.containerCornerInsets.topLeading.height)
                .ignoresSafeArea()
        }
    }
} 

When I added this with the red rectangle here is what I get for the red rectangle.

Then when my iPad app goes full screen I get