Side menu does not appear if cut off on small screen sizes

[faucomte97]

Describe the bug
If playing Rapid Router on a screen that doesn't fit the whole side menu, what should be visible when the user scrolls down to reveal the rest of it isn't there, instead it is just a white bar.
This causes issues like the "Exit" button not being accessible.

To Reproduce
Steps to reproduce the behaviour:

1. Go to level 1 (or any other level).
2. Resize the browser window so the "Exit" button no longer fits within the window (ensure an extra scrollbar appears on the far right).
3. Scroll down.
4. Observe that the "Exit" button (as well as the coloured background) doesn't appear - a white bar is shown instead.
5. Additionally, notice that while the "Exit" button isn't visible, it is still there and can actually still be clicked!

Expected behaviour
Whatever section of the side menu doesn't fit the window should be visible when scrolling down.

Screenshots

Additional context
Originally observed on Chromebooks within a class workshop.