The panel
The viewer shows the full text of the document as plain text in a single panel headed BizTalk ESB Viewer, using your VS Code theme's colors. Long lines wrap, so wide itinerary definitions can be read without scrolling sideways.
The content is shown exactly as it is stored in the file. Nothing is reformatted, reordered or removed.
Refresh
The panel updates whenever the document changes, for example when you edit the same file in a text editor tab next to it, or when VS Code reloads it after another tool writes to it.
Editing
The viewer is read-only. To edit, reopen the file with VS Code's text editor (View: Reopen Editor With…→ Text Editor), or open it in a second editor group. The viewer picks up your changes.