So far I was using Notepad++ but it's lack of proper support for the
MetaSL language is a letdown. Handling several shaders in a single
file is a pain in the ass as the fold/unfold functionality doesn't work
very well. I'd really like to have a "table of contents" kind of listing
of shader definitions in the currently open file among other things,
so I'm looking for a better application.

What software do you use for writing MSL shaders?