This is the cause of [the Gaia-ui test failures reported by TravisCI](https://travis-ci.org/mozilla-b2g/gaia/jobs/18362136). Without setting the `z-index`, the banner element will block mouse clicks to the bottom of the alarm list.

I can think of two solutions:

1. Leave the `z-index` declaration as-is
2. Add a `display: none;` declaration here and `display:block;` declaration to `#banner-countdown.visible`

`2` is untested, so I'd lean towards `1`