Webstorm missing multiple project windows option

I'm trying to have multiple project windows open in Webstorm for Mac, and the option appears to be missing in Preferences:

https://www.jetbrains.com/help/webstorm/opening-reopening-and-closing-projects.html#ws_opening_multiple_projects

All it shows under Appearance & Behavior is Colors and Scope.

I'm running version 2018.2.5 on Mac OS High Sierra.