Window resizing works perfectly fine on macOS if you use the native AppKit framework. It’s only when you try to embed a whole different world (Qt, GTK, Flutter, Electron) in your window via an opaque OpenGL/Metal surface that things break/lag/distort on resizing the window, which again is another reason to stay away from these cross-platform GUI shims.

I don’t really see the reason for multithreaded UIs either. A single UI thread works perfectly fine if you move your business logic off to a separate thread. Most people are just too lazy to do that, and start developing their app with the business logic running synchronously on the UI thread; which works for a while, but eventually reaches a point where it starts blocking the UI event loop; then they try bandaging it by moving certain pieces to a background thread and they shoot themselves in the foot with thread-safety issues. The way to solve this is not by making the UI framework multi-threaded… Simply be disciplined with UI ↔ business logic separation from the get go and all these problems go away. Yes, I know it’s so tempting to call the database directly from the button click handler - but don’t do it.