Link editor proposals in the tools column scroll around over the rest of the tools column items with the (scrollable main) content while the rest of the tools column items stay fixed.
(Sometimes there's just something that looks like a lonesome <hr> element of the same width being scrolled around. I don't know yet how to reliably reproduce that but I guess it's the same thing.)

Tested with Mozilla Firefox 38

Update – detailed instructions:
Mark a word in the translated-text column that is not linked yet (but is likely to have a wikipedia article available[?]), click "link to other page" in the tools column. A list of proposals pops up underneath the search field in the tools column. Scroll around and watch these list items being scrolled around.