To address this issue, there is now an option to make GAF fixed-width.
If you load the site in desktop mode, you'll see the "Change layout" option in the footer next to the button to change themes. You can use this to toggle between full screen or fixed width mode.
This value is stored in a site cookie
, so it'll be specific to a single device (rather than an account setting that would change your layout on every device you're logged into).
As always, thank you for your feedback and continued support.