In the old wikitext editor in Monobook skin, the layout of the edit mode switcher is broken:
The active area of the switcher extends over it's visible area too far below, and when the menu is open it is placed too far below by the same amount.
This doesn't happen in VE, nor does it happen in other skins, so it seems like a problem specific for Monobook/Apex and the WikiEditor toolbar.