tl;dr the answer is
How can I re-open the Input Assistant after I've mistakenly closed it?
E.g. Write the command
Plotand press Esc. Now the assitant is gone. How do I open it with a shortcut?
There has to be some shortcut to open it again, right? In many IDEs
Ctrl+Space is the standard shortcut for calling the autocomplete-y thingies but it doesn't work here.