Issue
This is an extreme case and should not be prioritized. When a Guest enters an extremely long username, the navigation bar at the top disappears entirely from the user's view.
Here is what happens when I enter an extremely long name:

Here is what it looks like normally:

Solution/Potential Fixes
Instead of collapsing the navigation bar towards the right, I think it would be better to collapse the navigation bar towards the left.

Currently, adding a longer name pushes Home, Library, and all other buttons to the right. I think that instead of moving home and library to the right, we can move the new user's name towards to the left, essentially only showing part of the user's name at once.

Another solution that I think is much simpler is to simply restrict the length of the username that a guest can type. This will prevent any navigation bar issues while not changing the functionality of the navigation bar.