Opera 9 and inline CSS popups

I'm trying to make a simple CSS-based popup. It works fine in Firefox 1.5 & IE 6, but Opera 9 has a problem if the "hover here" box is displayed inline. I haven't tested it in any other browsers.

The popup is obscured until you hover over the popup.
-display:inline- Hover Here This is the popup

It appears the popup needs text under it to display correctly.
-display:inline- Hover Here This is the popup blah
blah blah blah blah blah

But it works fine if it's not inline.
-display:block- Hover Here This is the popup

Floats work.

-float:left-

Hover Here This is the popup

blah