In the Scrivener 3 tutorial, in the “Comments & Footnotes” document, it says:
Tip: If the comments aren’t open in the inspector and you don’t want to show them, Cmd-click on a comment or footnote to open it in a popover instead of in the inspector.
However, that is not happening – the Inspector pane switches to the Comments & Footnotes tab.
I think you have to have an option disabled in Preferences > Editing > Options > Open Comments in Inspector if Possible – if that’s unticked, then cmd-click works, otherwise it opens the Inspector. Wonder if it’s a late change?
Probably needs a slight amendment to the Tutorial to make this clear?
It’s a mistake in the tutorial owing to changes made after it was written. The command should be Option-click, not Command-click. (It was briefly changed to Command-click because of a problem in macOS whereby Option-clicking wouldn’t work, but that was resolved in madOS so I changed the shortcut back, since Option-click is a more intuitive shortcut for this).
Freudian slip? 
Ha, it’s a good name for High Sierra. 
Right - so when I found that cmd-click works with the option off, it’s because it’s actually ignoring the ‘cmd’ part and just using the standard ‘click for popover’ feature.
Understood. Thanks.