Site width no more adapted to device width

I’m running my website https://www.mathcounterexamples.net with WordPress 4.1 and Mathjax-Latex plug-in 1.1.3. Since around a week, the rendering on my iPad or my Android Nexus 5 is wrong in horizontal views. In particular the website view is no more adapted to the width of the device. It is schrinked. You can see the issue by going to the website home. If you go to the menu Topology, it works fine.

When I deactivate Mathjax-Latex plug-in, the site width is a bit wider, but still not adapted to the device width.

Do you have a clue of what can be wrong?

Thanks, Jean-Pierre.

PS: i posted also a thread on Stck Overflow.