Microsoft has open-sourced XAML Studio, a visual development tool for building Windows application interfaces. The project is now available on GitHub under the MIT license and has joined the .NET Foundation as a seed project.
It is a rapid prototyping tool that lets developers design and test Windows app interfaces using XAML, the markup language for WinUI, before they integrate the work into Visual Studio.
Think of it like Qt Design Studio for Qt applications, but specifically for Windows apps. It provides live editing, meaning you can see your interface changes in real time without needing to recompile your application.
I imagine that the XAML output by this app would work with Uno Platform. If XAML Studio itself can be made cross-platform, this would create a fairly nice cross-platform C# story.
https://platform.uno/platform/