Hidden admin menu elements still accessible

[bull5-i]

When viewport width is less than 767 pixels, the admin menu automatically hides. This is done by setting the #column-left div opacity to 0, however all buttons are still accessible, although not visible.
For example the dashboard menu item will overlay roughly the first two characters of the page title, so when clicked on that location it would take you to the dashboard page. This should not happen when the menu is hidden.

Fix: Additional CSS display:none property should be added when hiding the menu so the menu elements would not be accessible when invisible.