static/ext/src/ux/dd/CellFieldDropZone.js
function(target, dd, e, dragData) {
        if (this.dropOK) {
            var value = dragData.field.getValue();
            target.record.set(target.fieldName, value);
            this.onCellDrop(target.fieldName, value);
            return true;
        }
    }
function(target, dd, e, dragData) {
        if (this.dropOK) {
            var value = dragData.field.getValue();
            target.record.set(target.fieldName, value);
            this.onCellDrop(target.fieldName, value);
            return true;
        }
    }
Process the drop event if we have previously ascertained that a drop is OK.