Moderator, I apologize for offending you by multiple posting - that is NOT my intention, but it MIGHT be explained by the fact that I am not being alerted by email about replies; moreover, I could not find this post in the forum except by searching on MY posts.
As for "I'm unable to find any "Snap" option (ticked or unticked) in the context menu of any Window Title bar in Windows 10 1703" I think '3rd party program' has to be the explanation - the 'culprit' being Actual Window Manager which adds wonderful enhancements, but apparently has the BUG resulting in the complaint of my post. I KNOW it introduces unwanted transparency by some miskeyings; SNAP misbehaviour may be another unintended consequence.
As for Geek - an understanding of SNAP may be gained by looking at it in Settings>System>Multitasking where the options are more or less explained.