27 December 2020 8 PM
The good news:
I've figured this out. See: https://github.com/geany/geany/issues/2708
The bad news:
The bug is completely deterministic and much worse than I thought.
I won't trouble you with the details of false correlations, mis-directions, confusion and frustration that has beset me the past few days.
Suffice it to say that if we hadn't butted heads over our very different experiences with geany, I never would have looked at the things that I needed to look at to resolve the ambiguities.
Long story short: we are both right.
You are correct that geany is designed for single execution.
I am correct that all I have ever experienced is a wonderful development environment for multiple simultaneous processes.
I am now closing this issue, which is only useful in the sense that it led to clear understanding as expressed in: https://github.com/geany/geany/issues/2708
Resolving https://github.com/geany/geany/issues/2708 is up to you and the geany developers. I can only hope you take heed of my concluding remarks. As of now, I wash my hands of this and return to my other tasks, since I now have a deterministic understanding of how to control the anomalous behavior of geany that actually is not anomalous, but only very complicated.