combobox = e.sender.input
dropdown = e.sender.filterInput

What i did was the code below. But with ComboBox theres a downside (see my another post in this "idea"/issue).

function onFiltering(e) {
    console.log('filtering');

    e.preventDefault();
    var v = e.sender.input.val();

    var newFilter =
    {
        logic: 'or',
        filters: [
            { field: 'Nazov', operator: 'startswith', value: v },
            { field: 'Special', operator: 'startswith', value: v }
        ]
    };

    e.sender.dataSource.filter(newFilter);
}
"

I've achieved accent folding with an additional property where i've stored text without accent. The downside of it was that auto-suggest and highlight first somehow did not behaved as they should.

By default neither selectize.js supported accent insensitive search, but having the possibility to override the scoring function, it was easy to implement...

Please add a function to override default client side search, so we can add accent insensitive search.

I moved from selectize.js where some function was easier:
1) template (f.e. when new item is entered - added just a function: return '<div class="create">Other value: <strong>' + escape(data.input) + '</strong>&hellip;</div>';
2) score: function (search) { ... }
3) ability to define opt groups and depending on the value use an other string/template to display)