I believe my confusion was due to the fact that the docstring of org-startup-folded is not particularly precise regarding to its possible values. On the one hand it seems to be a toggle, but its initial value is nevertheless showeverything and not t. On the other hand it states that "This can also be configured on a per-file basis by adding one of the following lines anywhere in your buffer..." but it's not clear whether these values may also be assigned directly to the variable. If you do that, then inconsistencies appear: when they are options in a buffer, showall and nofold are both mapped to nil, fold and overview are both mapped to t; but when they are directly set this mapping obviously doesn't take place. Following a suggestion I'd read elsewhere, I set org-startup-folded to fold (and, later, to overview), and this is not really calling org-overview but instead ends up in a catch-all clause that hides drawers. By the same token I could have set org-startup-folded to foobar.
The point still stands that showall and contents fold drawers but overview (aka fold) doesn't fold drawers, which seems extremely odd to me. Best regards, Carlos
