Open folds in autocmd makes vim start at strange lines

Dear all,

I wanted a way to make vim, when opening a file, open all folds it
contained. But *only* when opening the file: if I closed some folds on
the original file and then switched to another window or buffer, when
returning to the original one the folds I had closed should remain
closed. The only way I could solve this was by putting this in my
.vimrc:

autocmd BufWinEnter * :call ExpandFoldsOnOpenFile()

function! ExpandFoldsOnOpenFile()
if exists("b:did_initial_fold_expansion") | return
endif
let b:did_initial_fold_expansion = 1
normal(zR)
endfunction

Of course I did *not* want to do this for every file, but before I could
move that piece of code out of .vimrc, I noticed something strange: when
opening a lot of files, even ones with no folds, vim would *not* start
in the file's first line. The most notorious case was mutt: vim would
start in the first non-blank line (either the signature line or the
"Reply Original in..." line if replying to a previous mail).

Now my question is... *why* does this happen? I mean, if I open a file
with no folds, and do "zR", the cursor stays in the same line. I've read
the docs on folds, but I could not an answer. Ideas anyone?

Thanks,
--Óscar

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