I was just hovering over the '...' next to the backlinks on a page on http://ikiwiki.info/. In terms of the size of my browser window, this was towards the bottom-right of the screen.
When I hovered over the '...', the additional backlinks float appeared. This caused the page length to grow down, meaning a horizontal scrollbar was added to the page. This meant the text reflowed, and the '...' moved outside of my mouse pointer region.
This caused an infinite loop of box appears... text moves, box disappears... box re-appears.. which was not very visually pleasant.
In general I think that the onhover float is a bit of bad UI. Even a truncated list of backlinks looks cluttered due to there being no delimiters. I moved to having an always-complete list of backlinks and having them as LI elements inside a UL to make it look neater, although I appreciate that would make some pages very long indeed.
How about doing something a little like toggle for the excess items instead?
An additional, related issue: if the box expands beyond the bottom of the page, you might move your mouse pointer to the scrollbar in order to move further down the list, but of course then you are outside the hover region.
I agree, browser handling of this CSS is often not good.
Another idea would be to make the "..." a link to the ikiwiki cgi. The cgi could then have a mode that displays all the backlinks of a page in a list.
Yet another idea: Find some more refined CSS for handling a variable size popup.. --Joey