Wow thanks for the reply. I noticed it mainly (possibly exclusively) occurs when the navbar comes into view, or goes out of view (so scroll up, release,then scroll down). It gets annoying because the nav bar reappears if you switch tabs, or leave chrome and come back later.
We're actually having a hard time reproducing (though we've received some interesting screen shots). Our nexus 4 and Nexus 7 look ok. Rest assured we'll be acquiring more devices and will try and fix.