On Chrome and on certain conditions of screen size, window size, browser zoom, OS scaling factor, we can see a very thin line of header on top of viewport when sticky header is hidden.
Header is actually hidden with transformY(-100%), and when you change it to -101%, the thin line disappears.
It is a known rounding issue of fractionnal pixels with Chrome.
I guess 100% is not enough to hide header in all cases
I’m not sure you need to care for this, actually…
If no end user reported it, I guess it is because I spend a lot of time in debug window with different zooms