inline-block baseline

What is the baseline of an inline-block with a flexbox in it?  I see 3
possibilities:

1) The baseline is the last line box in the flexbox.  This seems to be what
the last sentence here says: http://www.w3.org/TR/CSS21/visudet.html

2) The baseline is the bottom of the flexbox.  This would match the
behavior of tables.  Example: http://jsfiddle.net/84FRJ/

3) The baseline is the baseline of the flexbox.

This came up in WebKit because the <input type="month"> picker uses a
horizontal flexbox to hold the date components and center them vertically.
 If the expected behavior is 1 or 3, the input will baseline align
automatically so I would probably lean towards either of those.  I'm not
sure what use cases there are to prefer one over the other.

Thanks,
tony