I tried using the French WIkipedia and narrowed my browser window to about half the screen width (I often do this if I wand to see two windows side by side). Rather than a nice responsive behaviour it keeps the fixed width and introduces horizontal scroll bars which is not great for UI. Image attached.
Topic on Talk:Reading/Web/Desktop Improvements/Features/Limiting content width/Flow
@Salix alba thanks for pointing this out, and for the helpful screenshot. We are aware of this issue and are working towards removing the minimum-width that is currently there. Once we've collapsed the user links and made a few other changes we will make that update and the site will be nice and responsive : )