Slim sidebar is not useful because on mac the ancestry icon is lost. Does not matter what the expanded sidebar view you start in. As soon as contracted the icon is hidden and not accessible.
This has been reported. It will happen if you collapse the side menu when it is already opened. If you adjust the height of the window a tiny bit it will make both icons appear again. If you want to keep the the side menu collapsed all the time you can change the default under Settings, Program Settings, and Start with side menu collapsed.