Platforms - bottom hamburger menu disappears when screen scrolls down

[calumcraig]

Steps to reproduce:

1. Launch hub.safedev.org.
2. Go to platforms page.
3. Either launch in mobile browser or resize desktop browser so menus collapse to hamburger menus.
4. Click bottom RHS hamburger menu:
   
5. Scroll the page downwards.

Expected:
Pop up hamburger menu remains visible.
Actual:
Menu disappears from screen: