How to implement scroll bar in multilevel dropdown menus parent and child menus both?

I want to apply the scrollbar in my parent and child dropdowns, my dropdown menus contain a lot of options for that scrollbar is mandatory, How can I do that?

Here is the reference image: dropdown-image

Here is the reference fiddle code , I want to apply scrollbar in both menu "https://jsfiddle.net/Nimra/ap7mhskn/4/"

I've experimented with various options, but I can only achieve a single scrollbar at a time, either in the parent or the child. In contrast, I require both to have their scrollbars simultaneously.

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