Code edit mode for UI Builder (or just open with default code editor button)

Hi. With current UI Toolkit, double clicking UXML file opens UI Builder. But UI Builder does not have code editor, so you can't edit UXML code directly from Unity editor. If you want to do, open Windows Explorer (or Finder on Mac), navigating to target UXML file, and open it with your own text editor.

See Visual Studio's XAML designer, it provides both visual designer mode and code editor simultaneously....

Code edit mode for UI Builder (or just open with default code editor button)