The 'panelHeight' property is defined in combobox not in datagrid. To define a combobox editor with auto drop-down panel height, try the code below:
$('#dg').datagrid({
columns:[[
{
title:'...',
field:'...',
editor:{
type:'combobox',
options:{
panelHeight:'auto'
//...
}
}
}
]],
});