Fix Avalonia IDE Code Tabs

I have built a small C# IDE with Avalonia where users can either type code directly or double-tap a method in the tree view and edit that method visually through a CodeEdit UserControl. After refactoring the original TabControl into its own UserControl (so multiple CodeEdit instances can sit in separate tabs), the tabs open but every CodeEdit surface is blank. No exceptions are thrown and the debug output stays clean; the method list itself still populates in the tree view exactly as before.

The task is simple but surgical: trace why the selected method’s code is no longer passed into the CodeEdit UserControl after the TabControl-to-UserControl move, and restore the previous behaviour. I expect you to diagnose the binding or data-context issue, patch it, and confirm that:

• Double-tapping any method still spawns a new tab.
• The corresponding C# source now loads into that tab’s CodeEdit instance.
• Existing functionality—block editing, saving, etc.—remains intact.

The solution should be delivered as a zip file.