Mobile menu is shown beneath page content due to incorrect z-index

Problem:

When the admin toolbar is visible the z-index of the main menu is changed. This is done to ensure the admin toolbar is shown on top of the main menu. However, this also causes the main menu to appear under certain page elements.

This happens in all browsers

Solution:

Don't try to change the z-index of the main menu when the admin bar is present, but instead properly update the admin toolbar's z-index.