Screenshot tells better.
Footnote popup does not have any minimal width, and if footnote link is located somewhere close to right border of the window, tooltip becomes very narrow, unreadable, sometimes out of the screen, so that horizontal scrollbar appears and page reflows.
For example, open http://www.dokuwiki.org/
find a footnote link, resize browser window so that link becomes close to the right window border and then hover it.
Tooltip should appear only in visible area, if it does not fit on the right border it has to be moved left, and if it does not fit in the bottom, it has to be moved up.
Also, there should be some minimal width for a tooltip, or it becomes tall and unreadable.