A new left-side window popped up in my zotero and I cannot remove it. It shows the nearly the same thing as the bar on the top, showing each file I open.

As shown in the images, it can be closed, but cannot be removed. How can I remove it? Many thanks!

https://s3.amazonaws.com/zotero.org/images/forums/u8556553/lww7yaazbnfffopfbem2.png
https://s3.amazonaws.com/zotero.org/images/forums/u8556553/fkvoemuswwh92koy2m5a.png