Prevent, suppress, avoid Navigation Pane display

I have read a zillion comments on this topic.  Many point to Acrobat Preferences settings.  Some of those are referring to extinct versions of Acrobat DC.  The only promise of certainty seems to be through hacking Windows Registries.  I am on a Mac.

This has bothered me for ages.  With some documents Nav Pane showed, sometimes not.  I could never discern a pattern.  Until now.

I isolated a known and proven and reliably No-Pane document.  I've built a whole, complex production, deliberately sticking to uncontamination of this Pure Document, taking care to not bring in pages from contaminated, known Yes-Pane documents, etc.  Things have been swell, over the course of production.  I have been delighted to not see the dreaded Nav Pane make an appearance.  I can live with it, during compositin.  For my readers, and for the document's graphic design sensibilities, the Nav Pane is death.  Here is what just happened:

After many thousands of 'Master' document edits, the Nav Pane reappeared.  I had duplicated the Master, to  TestDoc, to evaluate operation of some button functions, and a few simple lines of script.  The test went as I'd desired.  I went to bring some of those test features over to the Master, likely via copy/paste.  Lo and behold, I saw the dreaded Nav Pane showing in Test Doc.  I feared that reproducing said button functions/scripts would contaminate the so-far pure Master.  So I set about writing this post.

In the middle of writing this, it occured to me that I should go back and replicate the problem.  Took the same genesis version of Master that had spawned TestDoc, duplicated it to TestDoc2, copy/pasted fields and scripts from TestDoc to TestDoc2, expected TestDoc2 to become somehow infected with Nav Pane disease.  It didn't happen!

So I shall continue developing the Master.  But just so I can sleep at night, without anticipating its reappearnce in some way that can only be cured by rebuilding all, from scratch, the question becomes

1. What, in fact, and in truth, and for certain, causes the Nav Pane to show, or not?
2. How might one remove/eliminate its showing for a set of documents [all of TestDoc's family of a dozen or so variants] in which it appeared without explanation?

I know there are such things as Purification Rituals and Sanitation Measures, and such, often in the guise of Preflight, Print Preparation, and down-the-line necessities.  What in the world might manage [cuz it's clearly something in the document, not the user's keyboard/mouse, and Preferences unchanged during this 'infection'] the default visibility of the Navigation Pane in the Acrobat Viewer Window?