This might be interesting additional info: The same error manifests in a pure-Swift app that uses SwiftGL (an OpenGL wrapper) to create a single window containing an OpenGL-rendered triangle. There are no SwiftUI or any other UI components in the app at all. The project is basically this example app from the SwiftGL tutorial page. See the tutorial at swiftgl.github.io for more info; the app is the result of the tutorial's very first step. I'm running Xcode 16.2 on my late-2019 MacBook Pro on Sonoma 14.6.1. FWIW, I also needed to install CGLFW3 (which is required by the SwiftGL wrapper).

I haven't reported it yet -- honestly, I wasn't even aware of the Feedback Assistant. Nice! I've filed bug FB16759412. Thanks!

I also have the problem with Xcode 16 on Sonoma 14.6.1, and a Swift command-line app that uses SwiftGL. I randomly get 1, 2 or 3 instances of the executable. Most of the time there are 2 instances on my MacBook Pro, while on my Mac Studio there are usually 3. This is definitely a quality-of-life issue for me. I spent a half hour wondering why the debugger wouldn't stop at any breakpoints. I finally realized that the debugger was attached to only one of the instances. Now the work-around is to guess which of the app instances is actually attached to the debugger? Yikes!