Margin notes don't protrude bottom margin

[majjejjam]

Usually, the baseline of the font goes along the bottom margin. Thus, the descent of letters (for example ’g’) protrudes slightly past the margin. It seems this is not the case with marginfix. Notice the difference between the margin note (at left) and regular body text (at right).

[shicks]

I assume this is a result of https://github.com/shicks/marginfix/blob/master/marginfix.dtx#L647-L648:

\def\MFX@attachmargin{%
  \MFX@htdp\Mfx@marginbox}%
  \setbox\Mfx@marginbox\vtop{%
    \vskip\z@\unvbox\Mfx@marginbox}%
  % ...
}

The \vtop moves the reference point of the margin box to the top, which I assume was important for proper alignment (though it's been a decade and I don't remember exactly why that was). Unfortunately, that means that the depth no longer corresponds to the depth of the bottom box, and so we've lost any information about descenders.

I don't know if the main column's box still has its depth at this point (I would assume not, or else the \vtop presumably wouldn't get the job done?) - if it does, then we could use that to line it up. Otherwise, we might need something more sophisticated.

I don't have the bandwidth to look into this further, but there's a start if anybody wants to give it a go. I'd accept a PR.