property TRANSFER_OPERATION_BLOCK

This property returns transfer operation enumerator value for block. This operation is used to block default drag handler. (such as MSHTML's default drag handler)

function onDragOver()
{
    var event = shell.currentEvent;
    if (event)
    {
        event.canDrop = 0;

        if (event.sourceData && event.sourceData.hasFormat("application/x-file") )
        {
            var ii = 0;
            var validdata = false;
            var path = event.sourceData.getStringAt(ii, "application/x-file");

            if (path.length)
            {
                validdata = true;
            }

            while (path && path.length && validdata)
            {
                validdata &=  (IsTextURL(path) || IsImageFile(path) || IsHtmlFile(path));
                ii++;
                path = event.sourceData.getStringAt(ii, "application/x-file");
            }

            if (validdata)
            {
                // TRANSFER_OPERATION_BLOCK - blocks handling by MSHTML's default drag handler
                event.operation = appUtils.TRANSFER_OPERATION_COPY + appUtils.TRANSFER_OPERATION_BLOCK;
                event.canDrop = 1;

                var editor = scene.getBoxById("editorID");
                if (editor)
                    editor.setCaretPos(event.frameX, event.frameY);
            }
            else
            {
                event.canDrop = 0;
            }
        }
        // else bubble event to the default handler for dhtml control
    }
}