The usoft.support info didn't shed any light on my situation.

Attempt #1:
Put the port OPEN and CLOSE inside the polling loop, so that the OS
isn't hampered in its "Doing" by my holding open teh port port.
Well... it still won't service the mouse click to abort the polling
and now... I have a problem  because closing the port will no longer
capture serial data and when re-opening it, I get partial (a.k.a.
missed) messages.
The 300 DoEvents in a tight FOR:NEXT loop looks pretty servicable
right now.