I have Chrome set to resume where I left off (which may have been the cause of what just happened).
When viewing the forum for the first time today, its appearance had stretched to fill the entire screen, rather than it being as narrow as it is now and has always appeared to me since migration. As soon as I selected a menu it shrank back, so its appearance I saw and liked very much may just have been a glitch with Chrome picking up an open tab for the first time in a browsing session (similar happens with other websites). I don't see any means to adjust the view so am putting this down to a quirk, but having now seen how threads appear when filling almost the entire screen in landscape, I prefer it to the restrictive grey panels either side of a thread's content.
Can I adjust this myself? No doubt this has already been covered but I've not been paying attention.