Re-posting here because I do think that this is a bug.

tl;dr: "Vim is started with -S" is not, but it should be treated like "Vim is started with a file name argument" as far as viminfo-% functionality is concerned.

Countless times I would find, in a Vim session that I had started via vim -S Session.vim, some :buffers that I did not expect to see, because I had never opened those buffers in the session I had created via :mksession.

Today I've finally decided to go to the bottom of the matter, and it turns out that the problem was that I included % in viminfo:

set viminfo='50,\"100,:1000,%,n~/.viminfo
"                           ^
"                           |
"                           +-- culprit

Probably I've just copied that from somewhere, in the past, and I can probably just remove it.

However, based on :help viminfo-%, I wonder if it is intentional at all that that % entry in viminfo affects Vim sessions opened with -S. Indeed here's the text from the linked documentation:

When included, save and restore the buffer list. If Vim is started with a file name argument, the buffer list is not restored. If Vim is started without a file name argument, the buffer list is restored from the viminfo file. Quickfix ('buftype'), unlisted ('buflisted'), unnamed and buffers on removable media (viminfo-r) are not saved. When followed by a number, the number specifies the maximum number of buffers that are stored. Without a number all buffers are stored.

The part I've emphasised, I believe, maps to the intuitive behavior that one would expect:

• if I have no clue what to open, please, restore the buffer list,
• but if I do know what to open, please, don't clutter my buffer list.

In line with that, I would expect that opening Vim with -S would count like "I do know what to open", i.e. like "Vim is started with a file name argument", as far as restoring the buffers is concerned.

Does anybody agree?

Originally posted by @Aster89 in #19816