Just a heads-up to anyone who might find a different appearance on their talk pages:
As announced at w:fr:Wikipédia:Le Bistro/2 juillet 2020, the Reading/Web/Desktop Improvements project is planning to implement a fixed width screen size. This kind of change always takes a few days to get used to, and there's some research behind it, but if it's just not working for you, then @SGrabarczuk (WMF) has promised me that there will be a way to opt out of this via preferences. If you have questions, suggestions for a different default width, etc., then please go to mw:Talk:Reading/Web/Desktop Improvements.