HTML&CSS 드롭다운 코드예시:

https://velog.io/@kauthenticity/HTML-CSS-%EB%93%9C%EB%A1%AD%EB%8B%A4%EC%9A%B4-%EB%A9%94%EB%89%B4-%EB%A7%8C%EB%93%A4%EA%B8%B0

https://www.blueb.co.kr/?r=home&c=2/31&uid=4124&page=view
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https://codepen.io/suez/pen/gPRjBo
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https://www.blueb.co.kr/?r=home&c=2/31&uid=4124&mod=simulation&iframe=Y

리액트 드롭다운 코드예시:

https://codesandbox.io/s/yeof1

https://codesandbox.io/s/dropdown-menu-jzldk
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방법: https://letsbuildui.dev/articles/building-a-dropdown-menu-component-with-react-hooks

리액트-MUI 드롭다운:

https://gareen.tistory.com/2
https://mui.com/components/menus/

리액트-부트스트랩 드롭다운:방향설정:

https://react-bootstrap.github.io/components/dropdowns/