Well, there is something more that can be said about the behavior of my mouse pointer.
I did a full clean reinstall (deleting settings everywhere, etc.)
Now, with Full Row Select
either OFF or ON, when I repeatedly click in the white space after a filename and drag as if I wanted to select a bloc of filenames, the response is erratic, but never the one ecpected. Sometimes, the copy/move pointer tip will appear, and sometimes not (for the very same action, mind you). In either case, the dotted rectangle indicating the bloc selection area never
appears, the pale blue rectangle supposed to indicate the files selected remains fixed on the initial filename at the end of which I tried to anchor the selection, and the even paler blue rectangle around the filename under the pointer follows it as if the mouse was not clicked and held.
I hope this detailed description helps. But if this behavior is not reproducible at your end, I realize I'm in a pretty dark place. So let me thank you right now, and no matter what, for your excellent response to my call. I just hope that, eventually, with some obscure update, it goes away.