Not sure that's quite what he meant. It's been ages since I last did anything with website development so I forget the terminology, but basically what I pictured when I read his post was the sidebar not scrolling as you scrolled through a topic. Like if you look at the background image on https://www.purediablo.com/, scroll up and down and it just stays in place.
Also, on a subject related to this change, sometimes I like to use the split screen feature when browsing the web. (Here's a video just in case anyone doesn't know what that is.)
Anyway, when I do that now, the forum threads are squeezed down to the point where I feel like I'm reading top to bottom instead of left to right. There's like maybe three to four words per line. I imagine that means it's even worse for mobile devices.
@Glurin are saying you wanted the sidebar to stick as you scroll? As for width, the sidebar can be slid away when you wanted to use split screen. That may be something we change for PurePremium only but right now we are just trying to get a feel for what works.