Inserted documentDrag in ModifyFeature so that you can drag vertices over the viewport of the map in analogy to the DragFeature-Control. documentDrag is internaly passed to the DragFreature that's used by ModifyFeature.

lib/OpenLayers/Control/ModifyFeature.js

@@ -22,6 +22,12 @@
  */
 OpenLayers.Control.ModifyFeature = OpenLayers.Class(OpenLayers.Control, {
 
+		/**
+     * APIProperty: documentDrag
+     * {Boolean} If set to true, dragging vertices will continue even if the
+     *     mouse cursor leaves the map viewport. Default is false.
+     */
+		documentDrag: false,
     /**
      * APIProperty: geometryTypes
      * {Array(String)} To restrict modification to a limited set of geometry
@@ -245,6 +251,7 @@ OpenLayers.Control.ModifyFeature = OpenLayers.Class(OpenLayers.Control, {
 
         // configure the drag control
         var dragOptions = {
+						documentDrag: this.documentDrag,
             geometryTypes: ["OpenLayers.Geometry.Point"],
             onStart: function(feature, pixel) {
                 control.dragStart.apply(control, [feature, pixel]);