tooltip turns into a black rectangle when QToolButton uses a stylesheet "background-color: transparent"

Steps to reproduce:

• run the sample attached
• hover the mouse over a tool button -> tooltip
• move to another tool button to see its tooltip
• keep moving between the buttons and watch the tooltips
  EXPECTED: The tooltip are shown correctly with a transparent background.
  ACTUAL: Sometimes the tooltip is a black rectangle (see the 'qtbug68287.png' attached)