Top navigation sometimes on two lines

How to reproduce:

• Login with a user that has many tabs. If necessary open courses until you see the "more" menu
• Resize the browser to make the gap between the more menu and the chat as small as possible. Ideally one one pixel gap (just before the next item pops into the more menu)
• Open a group with many users
• In the group, go to admin in menu and make sure the window is tall enough to not produce a vertical scrollbar
  => no scrollbar visible, menu on one line
• In the admin menu, select the "members" tab
  => scrollbar visible
  => NOK: "more" menu rendered on second menu line

Expected behavior:
Top navigation should never be rendered on two lines.