Small Autocomplete visual improvements

There are couple of visual issues with autocomplete
- On drop-up behaviour downward selection is visible for a short time until it flicks to upward, which looks like it jumps from down to up
- Drop-up misses the top border on the items list box
- If there is no data to show yet, activated autocomplete shows a single line below the component

Tested with Chrome, OS X.

Start by locating the autocomplete component and its styling in the repository, then reproduce the drop-up and empty-state behavior in Chrome on OS X. Done means the drop-up selection does not visibly flip, the items list has its top border, and an activated empty autocomplete leaves no stray line.

Written by the indexing model from the issue text.