Navigation menu not showing

My site has many pages reached via a series of menus. Until a few days ago, these were on the left side of each page and would be displayed depending on the page name. No problem, worked for years.

Now the menus show up in a small block in the upper right corner and replace the main contents when selected.

This only happens of some desktops – works fine on others.

The theme is twenty-fifteen. The desktops are all Macs, and the browser is Chrome.

I have tried restoring from before the problem started, but the problem persists.

It appears to “think” my desktop is a mobile device, but I don’t know how to tell it the “truth”.

Thank you

The page I need help with: [log in to see the link]