There are just a lot of things to consider/recreate:
All of these thing must be recreated. A lot of effort and possible breaking bugs just to add a custom experience (not better for all) and a custom style. In design systems these things are considere UX breaking.
BTW, i agree with you that this operation must be done if it provide a real value, but at this point, it's not a custom dropdown, it's just a new component. 👍🏻 Cheers.
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