Menu items move slightly when selected

When a menu item is selected in the menu editor, the item moves slightly to the left when clicked. That should not happen: just selecting an item should not visibly move it.

Steps to reproduce:
1. login to the archetype CMS
2. navigate to the only channel
3. click 'Edit Channel'
4. click 'Edit Menu' to edit the main menu of the site
5. click the 'news' menu item, without moving the mouse between the mouse-down and mouse-up action

Expected: the item becomes selected without moving visibly
Actual: the item becomes selected, but moves slightly to the left on mouse-down, and back to the right on mouse-up