Sampler tooltip relative to mouse pointer

If you use the vaadin sampler application for the tooltip example you can see two items.
If you move your mouse over one item and wait the tooltip pops up and it’s show underneath the item you point at.
If you move your mouse back and forth over the items you can see the tooltips have each time a different offset and even can overlap the original item you are pointing.
Is this a bug or is there need for a special implementation to circumvent this problem?