Created attachment 508 [details]
Proposed fix

While investigating a related issue (https://bugzilla-geant4.kek.jp/show_bug.cgi?id=2082) I bumped into an obscure problem that might cause undesired behaviour.

Static variables which require the G4UImanager in their constructors might become instantiated before main starts. The two that got instantiated for me were these:

G4PlotManager::fgPlotParameters
G4ParticleHPManager::instance

I was able to trigger this issue using the Catch2 framework (https://github.com/catchorg/Catch2) and the following source file:

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
#include <catch.hpp>

#include "G4MTRunManager.hh"

TEST_CASE("run 1")
{
    G4MTRunManager man;
}
TEST_CASE("run 2")
{
    G4MTRunManager man;
}
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

All the necessary files can be found here:
https://gitlab.com/loximann/test-runmanager

See the attached file for a patch with the proposed fix. The fix consists of declaring the static instances inside functions, so they are only instantiated when the functions are called for the first time. I avoided renaming and reformatting for clarity, so probably additional changes would be needed to conform to the Geant4 conventions.