I downloaded 1.8.506 on a Windows machine. Indeed, it produces the aberrant pixels.
Then I tried my current development version on that Windows machine. Pixels are gone.
The thing is, I have no recollection of any changes I made between 1.8.506 and the current dev version that would have made a difference, and hence I've been trying my hardest to replicate the issue on a version that is - apparently - not exhibiting the issue.
Either some code fairy fixed the issue in the meantime, or my sleep-deprived brain made a fix for this and can't remember. I'd like to think it was the former, but it's probably the latter (which is a worry).
At any rate, I will release a new version before the weekend. Thanks again for helping me ferret out the issue!
Not too hard though.