I've still no idea what's causing this behaviour on my Win 10 PC.
BTW, I note that unlike 1.8.19 the IDE 2.3.6 menu is no longer accessible by the familiar KB shortcut method, such as <Alt + F> for File, etc.
I'm greatly appreciating some of the 2.3.6 advantages, especially opening sketches much faster. But on balance I'm considering reverting.