A user tabs into a drop down menu and the menu opens up. By 508 compliance, does the tab go to the next item in the menu or is it solely just the arrow keys?
It is confusing because on government access board the left panel menu, which is not a dropdown, is only nav'd through the tab. Up and down keys do nothing. But then again, it's not a dropdown menu...so that is different?
I can't find any official documentation on it because it is so UX unfriendly.
I don't believe that this topic is specifically addressed by Section 508 or WCAG, however there are a number of resources on best-practices available.
https://a11y-guidelines.orange.com/web_EN/exemples/simple-menu/simple-menu.html
https://www.w3.org/WAI/tutorials/menus/flyout/#keyboard-users
To the best of my knowledge, the consensus is that users shouldn't have to traverse sub-menus to get to the next top-level menu item.
For this reason, it's typically recommended have the Enter and/or Space keystrokes trigger the expansion of the sub-menu. Once expanded, the up/down arrow keys should then be used to move between sub-menu items.