I have a strong memory of using an IDE that behaved like this - using size and font to communicate structure - at a job around 2005. I can't currently remember

I have a strong memory of using an IDE that behaved like this - using size and font to communicate structure - at a job around 2005. I can't currently remember the name but think it it started with 'source'? If someone else remembers this and the name I'd appreciate it. I could be remembering wrong too about how far it went with such decoration.