zia

The calendar and links beneath it were in a div that was asking
to claim twenty percent of the browser window's width.

The supposedly floating right-hand-side "rest-of-the-window"
(no percentage claimed) was overlapping the thin borders around
each item. Some experimentation led me to believe
that the content of the inner boxes affects how wide Mozilla
wants to display those boxes — but that it does not account
for the thin border-lines when sizing the other column.

Workaround for now: change the twenty percent claim to 23%.