As the code above mentioned, reset the column width to its percentage value after resizing a column, you can easily solve your issue.
$('#dg').datagrid({
onResizeColumn:function(field, width){
var state = $.data(this, 'datagrid');
var opts = state.options;
var col = $(this).datagrid('getColumnOption', field);
col.width = width/(state.dc.view.width()-opts.scrollbarSize)*100+'%';
$(this).datagrid('resize');
}
})