[macOS] Tooltip shadow disappears if mouse is moved quickly elsewhere to show another tooltip

Steps to reproduce:

• run the sample attached
• hover the mouse cursor over the first widget -> tooltip appears
• quickly move the mouse cursor on the other widget, and let it hover -> tooltip appears
  EXPECTED: Both tooltips have a shadow effect.
  ACTUAL: The newly opened tooltip does not have the shadow effect it should have (see 'qtbug_tooltip_shadow.png' attached).