While this commit achieves what the subject advertises (fixing the
input box's height to match the recipient drop-down), but it does
so by relying heavily on flexbox's behavior to manage the height
of the elements and vertical centering, where necessary, rather
than positioning hacks or vertical padding.
This commit also removes some additional styles that do not make
sense (e.g., `min-width: 0`) or that need not be set.