No tooltips are shown

With Chrome and Internet Explorer, very often no tooltips are shown. I am really wondering why I don't find this bug documented anywhere. It even occurs in the official vaadin demo http://demo.vaadin.com/sampler/#ui/basic-features/tooltip
So it has nothing to do with any addon or theme or whatever. It's a real bug occuring on every computer I tried with a windows os and the latest browser versions. It always works with Firefox.
What I figured out in Chrome is that obviously the mouse event eventually responsible for showing the tooltips is fired endless as long as your mouse curser is located somewhere above the browser window. That's why the scheduler which should move the tooltip to the mouse position is restarted again and again. But my knowledge about JavaScript is far too insufficient to figure out why this is happening.
So the tooltips are shown if one sets the tooltip delay to 0, but that's no nice workaround.

Is this a known issue? Is there any better workaround? I can't find anything...