Alright, I've beefed up our theme a little bit. This should help on a variety of devices.
If you are using a mobile device, if your total screen width is lesser than 980px the sidebar will disappear.
On desktops, if your screen width is lesser than 960px (half of the width of a 1920x1080 screen), the sidebar will be equally gone.
If the changes don't apply for you or the sidebar is appearing above the forum content, clear your cache or use CTRL+F5.
