If you accidentally open a this useless left panel in Safari (or somebody else did for you, or it was enabled in default configuration) it might be really hard to figure out how to get rid of it. There’s no close/hide control on it. You might think: go to the menu! But you don’t even know what it’s called! Glasses panel? There’s no menu entry for that. The pressure is high though: it covers good portion of your screen and stays open on every new tab you open. You might manage to resize it to zero but that does not get rid of it. Like a phoenix, it will ressurect on the very next tab you switch to.
Thx @apust for pointing this out.