Ok, so the do-while is run 13 times. If each of those is 0.3 seconds, it explains the thing.
And the reason it is run 13 times is, if I understand things correctly, because the memory of my Pro Micro is not large enough to fit the frame-buffer for the partial window in one go.
Correct?
But I just got a Pi Pico, so I might try that one instead then, to see if that one will work better.