At the same time, the navigation bar extends slightly and replaces the arrow down icon (at an item with sub-items) with an upstanding rectangular special character. After less than 1 second, everything is over and the page has been changed as desired.
Functionally everything is ok; only the shivering and resizing of the navigation bar are annoying.
The whole thing I could observe exactly the same with another GP installation.
Try using border: 0px, display: inline-block and more have not done anything.