Collapse menu - overflow-y / scroll

[JaySMa]

Hello..
I noticed that there isn't an overflow-y scroll when there are many menu items.

For example, I hit the DNNBooster.com website within Firefox and went into "Responsive Design Mode", opened the menu and expand "Features," and you can't scroll to items at the bottom of the opened menu.

I've tried a couple different things, but can get the scroll to enable.

Thank you for your time and attention..