             # try to get the initial location now.
             if p13n.get_location_requested()
                 p13n.request_location()
+
+        _add_mouseover_listeners: (markerClusterGroup)->
+            markerClusterGroup.on 'clustermouseover', (e) =>
+                @highlight_unselected_cluster e.layer
+            markerClusterGroup.on 'mouseover', (e) =>
+                @highlight_unselected_unit e.layer.unit
 
         effective_horizontal_center: ->
             sidebar_edge = @navigation_layout.right_edge_coordinate()

            # over in search results or otherwise temporarily in focus.
            @popups.clearLayers()
            parent = @all_markers.getVisibleParent unit.marker
            popup = unit.marker.getPopup()
            popup.setLatLng unit.marker.getLatLng()
            @popups.addLayer popup

        highlight_unselected_cluster: (cluster) ->
            # Maximum number of displayed names per cluster.

                maxClusterRadius: 30
                iconCreateFunction: (cluster) =>
                    @create_cluster_icon(cluster)
            @_add_mouseover_listeners @all_markers
            @popups = new L.layerGroup()

            L.control.zoom(
