Menu on mobile expands the viewport width

[lb-]

Steps to reproduce

• On a mobile device (or emulator) open any page in the Wagtail guide
• observe you cannot scroll past the edges on the left or right
• Now open the menu
• Expected - the viewport cannot scroll left or right
• Actual - the viewport can be scrolled to the left or right causing the content to move outside the viewport

Screenshots - see below how that bird logo is slightly offset when the menu is open as I'm able to scroll

Might get solved as part of https://github.com/wagtail/guide/issues/199