I am using the SafeWheels Theme and the Sub-Menu Being Cut Off

1. I do not want to use off-canvas menu for desktop view.

The SafeWheels template doesn’t use off canvas menu for desktop.
If you accidentally turned it on, you can turn it off at customizer > layout > off canvas panel.

2. I want to allow the user to scroll the SUB MENU, not the Menu.

Not sure what you mean, can I see your site?

3. I saw some CSS that was posted a year or two earlier:

This CSS is not from the template.

What CSS do I need to add to make only the SUB MENU scrollable?

Can i see your site?

My sub menu for my website has so many choices that it is longer than the entire webpage when viewed on some smaller screen dimensions such as a laptop.

in this case, I would recommend using mega menu plugin to create a mega menu.

Or

you need to give the ul a max-height value so that it can be scrolled. Try this CSS for desktop:

.main-navigation:not(.toggled) ul li.sfHover>ul, .main-navigation:not(.toggled) ul li:hover>ul {
    overflow: scroll;
    max-height: 300px;
}

this CSS for off canvas menu:

#generate-slideout-menu.main-navigation .main-nav ul ul {
    max-height: 300px;
    overflow: scroll;
}

Feel free to change the max-height values.

Try this CSS:

.main-navigation ul ul {
    width: 300px;
}

Glad to hear that 🙂