From the look of your gif, it looks like you're just barely scrolling back up for a few pixels between scrolls and the navbar seems to pop up immediately after.

Scrolling up gets it to pop out for me, but I need to scroll up a lot more than you do in order to replicate this (scrolling down normally looks fine on Linux/Chrome). Sounds like it might be possible they haven't noticed it and would appreciate a bug report. :)