Hi,

I think, if you want to change the way the grid is rendered, you're going to have to write a bit of dojo. I don't know how you want the nested list to display but, assuming you have a property on MySecondPropType called ItemName and you want the nested list to appear in the grid as a list of the ItemName properties, one per line, you could create a JavaScript file like this:

define([
    "dojo/_base/array",
    "dojo/_base/declare",
    "dojo/_base/lang",
    "epi-cms/contentediting/editors/CollectionEditor"
],
    function (
        array,
        declare,
        lang,
        CollectionEditor
    ) {
        return declare([CollectionEditor], {
            _getGridDefinition: function () {
                var result = this.inherited(arguments);
                // Apply custom formatting to the mySecondProps property here
                // Note the fact that property names are converted to camel case
                result.mySecondProps.formatter = function (values) {
                    return values.map(x => x.itemName).join('<br/>');
                };
                return result;
            },
        });
    });

If you save that to a location (e.g. /scripts/dojo/MyCustomCollectionEditor.js), you can then reference it from your list property like this:

[ClientEditor(ClientEditingClass = "/scripts/dojo/MyCustomCollectionEditor.js")]
public virtual IList<MyPropType> MyProps { get; set; }

Which should use the customised version of the collection editor to render the grid with the nested list neatly formatted.