It says to click the x to the right of the header bar and there is no x there.
Okay … so you don’t close the right panel?
You can, if you want to.
The right panel is one pane of a split editor. The single pane icon in the top right corner will return you to a single pane editor. But that’s a separate feature from the copyholder, which is closed with the ‘x’ that Bridey indicated.