This bug occurs on en.wikipedia (Wikiedior 0.3.1 + MW 1.21wmf9 (070f4b4)), but not with Wikieditor 0.3.1 + MW 1.18.1 (Version 113461).
Steps to reproduce:

1. Open any article for editing.
2. Click the chain icon to open the link assistent.
3. Type "a". A list of articles starting with "a" will open.
4. Press the down-key. The first item in this list will be selected and the input updated accordingly.
5. Press the down-key again.

Expected result: The second item should be selected.

Actual result: No item is selected, the input is reset to "a".

The same goes for the up-key, which selects only the last entry, but not the second-to-last.

Version: 1.21.x
Severity: normal