@Sylvester-Bullitt Thanks, but I already had the double-click speed cranked up to the max.
However, with the help of your comments (and those from other posterss), I think I finally discovered the root cause.
The various replies got me to wondering if the problem was outside Notepad++. I originally assumed NPP was at fault, since that was the only place I saw the symptom. But then I realized NPP was the only app where I insert characters inside text many times a day (because NPP is my only tool that lets me see the soft hyphen characters). This reminds me a little of the old adage “When all you have is a hammer, every problem looks like a nail.”
To make a long story short, I started Googling again, and stumbled across a post on another forum that hinted the problem might be hardware related. Specifically, it suggested that dirt, dust & other gunk accumulating inside the mouse might be interfering with its moving parts, causing the false double-click messages.
My mouse is corded (i.e., no way to open it, since it has no battery), but even a cursory exam of the outside showed a lot of junk filling up the outside seams, especially at the bottom of the two click buttons.
So I took needle and scraped out all the grime and crud I could see in the outside seams & re-tried the insertion point clicks. To my amazement, 99% of the problem went away! I speculate the 1% outliers are due to residual dust & dirt deep inside the mouse that I couldn’t see to remove.
So I’m going to declare victory on this issue! Thanks to all who took the time to reply. If nothing else, we came up with something that may help future troubleshooters.