I do not really understand what is the problem with flexible <select> implementation in browser.

In my Sciter[1] the following are supported ( see screenshot [2] ):

    <select type="dropdown"> - select with popup
    <select type="list"> - select list 
    <select type="tree"> - hierarchical select (a.k.a. tree view) 

and all of them may have

    <select multiple> - multi-select, value = array[]
    <select multiple="checkmarks"> - multi-select, value = array[]

select type="tree" may have also

    <select type="tree" treelines> - draw treelines 

<select> and <option>'s may have arbitrary markup inside (<hr>,<img>, whatever), even more any DOM element may have role="option" defined making it a selectable option :

    <li role="option">...</li>
    <option value="Sc"><code>Sc</code>Scandium</option>

And as a bonus, by declaring different flows in CSS in list you may have:

    select { flow:horizontal; } // horizontal select
    select { flow:vertical-wrap; } // multicolumn select

[1] https://sciter.com [2] https://sciter.com/wp-content/uploads/2024/01/select-variant...