Weird Safari behaviour! Today safari (v11.1) on Mac HighSierra 10.13.4, started to behave like crazy. For some reasons this is the resolution I get everytime I open the browser. Looks a kind of mobile resolution. I restarted, erased the history, changed from view --> font in, font out, but  nothing happens.

Here a screenshot:
 

What should I do? Now I am writing from Chrome.
I checked develop → User Agent and check if it's set to default. When I switch to some options, it reloads correctly and then goes back to the same view.