Godot 4.0 recently acquired multiple window support for the editor (which is built with Godot's UI and rendering system) and APIs for applying it to your own games
Just to clarify for those following along. Godot IDE is using the same UI components that you can use in your games. Godot 4 will support multiple windows. They're working on RTL I believe and it actually has a very robust code editor with debugger in the editor. Most of the points being raised here don't seem to be based on actual experience with the framework.