Usually that’s because you’ve posted a picture that’s too wide, but that doesn’t seem to be the case here (thanks for the link so we can check it out). Sometimes it happens because people paste things from elsewhere on the web that include formatting that makes it too wide as well, but you say you didn’t do anything. I am going to straight-out guess that it’s something on your computer. At least, this is what happens with mine sometimes: the screen size changes itself. I have to go to Settings and change it back.