I have a LibreOffice installation where I docked the Navigator on the left. I like it. On another document, it is always floating. I don’t like that. Try as I might I can not get the Navigator pane to dock.
click the three vertical dots in the right (or left) upper corner and select
Double click into the small area between window title bar and drop downs
Tested using LibreOffice
Version: 18.104.22.168 / LibreOffice Community Build ID: 47f78053abe362b9384784d31a6e56f8511eb1c1 CPU threads: 8; OS: Linux 5.3; UI render: default; VCL: kf5 Locale: en-US (en_US.UTF-8); UI: en-US, Calc: threaded
Hope that helps.