It currently is a OO.ui.Button, and its position is not handled by Leaflet.
Since we added the Kartographer hook to allow map customizations, developers can easily add controls to the map.
Leaflet is not aware that there is already a close button on the topright corner of the map, so will display the new control in the most top position, and this control will overlap with the "hardcoded" OO.ui.Button.
See a screenshot of the bug (taken from the Wikivoyage prototype T132971) :
