Reduce mathfield min height

[stefnotch]

This mostly prevents something I've asked about on Gitter

I realize that this is a bit of an application specific question, but is there a way of making the ML__fieldcontainer__field take up the size of its parent (ML__fieldcontainer)?
Currently, it's possible to have a field that's a bit smaller than its parent, leading to the user pressing in the mathfield and wondering why the caret doesn't get placed anywhere