Update 1: 2018/09/04 8:07am PDT
More confirmations have come in from commenters - thanks, Fredro and RobVdm! They've verified that this works as explained, but only for transit navigation. They're both using Google Maps 9.86.0.
The day is finally here. After whining, asking, begging, and praying, it appears* that the Google Maps team has listened and is working on implementing a way for you to launch navigation, continue using the app as normal, then go back to your directions without interrupting a thing.
* The asterisk is due to the fact that we've only received one tip about this, and we weren't able to replicate or verify it in any way.Read More
Google Maps tests an option to back out of navigation without stopping it, then resume thanks to a 'Current trip' shortcut was written by the awesome team at Android Police.