When your press 's', you should have a region selected, in order to stage that region - if your cursor is just sitting on a line without selecting part of it, you'll (probably) just get the whole line staged.

When I press s, the entire line is staged even if my region is a subset of that line.

...huh. I just tested this in my Emacs instance and it works as you describe.

I could swear I remember this working in the past, but I can find no hint of it in a few quick searches, so I'm likely mistaken.

I'm sorry for the misinformation.Thanks for saying something.

The second line you quoted tells you how.

It does not. As I said, pressing s does not stage less than a line for me, even if my region is a subset of the line.

When selecting only part of line, 's' will still stage the full line for me.

Indeed.

  cd $(mktemp -d)
  git init
  echo bar > file.txt
  git add file.txt
  git commit -m "init"
  echo foo_bar_baz > file.txt
  # select only "foo_" in magit-status and press "s"
  git diff --cached

will show

  diff --git a/file.txt b/file.txt
  index 5716ca5..2fd000c 100644
  --- a/file.txt
  +++ b/file.txt
  @@ -1 +1,2 @@
   bar
  +foo_bar_baz

rather than the desired

  diff --git a/file.txt b/file.txt
  index 5716ca5..2fd000c 100644
  --- a/file.txt
  +++ b/file.txt
  @@ -1 +1,2 @@
  -bar
  +foo_bar

I can't really imagine this feature working that well with just selection. Editing of the diff seems required.

I just updated magit, too. It seems to have been published 3 days ago.

..huh. I just tested this in my Emacs instance and it works as you describe.

I could swear I remember this working in the past, but I can find no hint of it in a few quick searches, so I'm likely mistaken.

I'm sorry for the misinformation. Thanks for saying something.