If a tooltip text of an taskbar entry is bigger than the tooltip text of another, you will see that the second one will get bigger tooltip area than should, with an ugly gray area above the text. To reproduce move the mouse first over the entry with the longer text, than move above the other one. If you move out and again above the one with the smaller text, the tooltip will appear at the correct size.
*** This bug has been marked as a duplicate of 157059 ***