You can already expand/collapse blocks by selecting the arrows in the gutter bar on the left. If you are next to an open { you can press Ctrl+q or Cmd+q on MacOS to toggle the fold.
I may also add a context menu option for this as well, but it would only work if the cursor is next to the opening brace at the moment.
Folded code will be unfolded when you click away from event sheet script blocks, as we convert back to a basic text representation in this situation for performance reasons. You can't collapse a whole event sheet script block at the moment but I think this is something we will do in future.
Text editor views won't clear fold state when you click away, but you will find code unfolded when you close & reopen files.
— Please add an item on the feature tracker if it doesn't exist yet. At the moment you can probably edit existing files with an external editor just by opening them with that editor. I'm not sure if we have a mechanism for auto reloading files that change on disk though, so you might want to ensure the project is closed in C3 when your editing it.