The Dbms_output message is not displayed in the SQL result grid editor when the trigger fires for the first time. When another statement is executed right after (e.g. a simple pl/sql routine), this causes the SQL-navigator to poll for dbms_output and the lost message then appears.
Steps to replicate:
1. Enable dbms_output on menu Session | Server Output
2. Create a table
3. Create a trigger for each row before insert or update that does "dbms_output.put_line( 'hello world' );"
4. insert a row in your table and commit
5. Do a select * from your table
6. Use grid edit to update a value
The string "hello world" should appear in the output window
Waiting for fix in a future release of SQL Navigator for Oracle.