Robin2:
And just to be clear (because this is what may be causing confusion) I started two completely separate instances of the IDE from the operating system. I did NOT create a second "window" from the first instance....R
Aha, yes, that would indeed be the problem. Opening a second window is not the same as opening the IDE a second time from the launch mechanism. I see no clear way I could have known that, however; there is no clear way of knowing that that is what one is doing, and there is no way of distinguishing between different instances once windows are open, other than the trap I fell into. Opening a sketch from the recent menu gives no indication that the new windows is tied to the one from which it was opened. I have three windows open at this moment, one opened from the other, and the third from the launcher icon, and there is no way I can identify that two of them are somehow linked.
I can see what Coding Badly was trying to tell me, but it only makes "sense" (lexically, that is: the behavioral difference is completely nonsensical to me) now I know there is a difference. I don't know how I could have known that beforehand. It is certainly not consistent with the standard practices deployed in windowing operating systems I've used over the past thirty plus years.
Thanks for making the distinction clear, Robin2.