put a load of margin on the bottommost sublist to increase its hit target size and avoid problems when toggling its hiddenness

src/skins/vector/css/organisms/RoomSubList.css

@@ -20,6 +20,11 @@ limitations under the License.
     width: 100%;
 }
 
+.mx_RoomSubList_bottommost {
+    /* XXX: this should really be 100% of the RoomList height, but can't seem to get at it */
+    min-height: 400px;
+}
+
 .mx_RoomSubList_label {
     text-transform: uppercase;
     color: #3d3b39;