Removing black box in left Table of Contents

Hi. How do I gt rid of the black box in the left Table of Contents pane in e-book reader? I can drag it to make it smaller but want to remove it entirely. See attached photo to show what I mean.

Edited: sorry, I just realized that the black box is reserved for the Search results so I'd like to change my question to: is there a way to make it auto-hide until I use search?

https://i.imgur.com/PQr70OP.jpg