When opening a PL/SQL file, the TOAD Editor opens Trigger in SQL-Style without the Navigator.
This behavior is a result of how it is initially selected, not an error in Toad.
The Navigator panel should open only if the saved desktop (SQL or PL/SQL for instance) had the Navigator panel saved as part of the desktop. In other words the same desktop should be shown as if a user simply created a new, blank PL/SQL tab.
Parsing the files on load, just to determine its file type, would be program-intensive, and is not being considered for future development work.