It turns out the answer to this was not all that complex: we had
two nested <label>s: one for the radio button and one for the Field,
which presumably causes both of them to generate click events and
focus something and what ends up focused is some kind of race condition
depending on, apparently, how fast you click.
Notes: Fix bug where the 'other homeserver' field in the server selection dialog would become briefly focus and then unfocus when clicked.
f9b45677d6