bug#10409: Minibuffer sized according to default face, not minibuffer face

[Chong Yidong]

>> The height of the minibuffer (in pixels) is set in increments of the
>> line height for the default face. If the echo area face (the one from
>> which all minibuffer faces inherit) is slightly taller than the
>> default face, this means that the minibuffer is one line high when
>> empty and two lines high if there's anything in it.

What is the "echo area face" you are referring to?

Please provide a precise step-by-step recipe for reproducing this bug,
starting from emacs -Q.