I used to do that (and it's part of why I put a double line between functions and classes). But over time I started to feel like those were the points where I should just refactor. (If the function can't be cleanly cut at those joints, in turn, I take that as a sign that the logic needs to be disentangled first.)

.Net languages have ‘regions’ that can collapse & nest, providing a high level narrative of file organization (and, in practice, display most files in a clean collapsed view).

I do it, but I also like to use headers.