What is this animated thin blue bar in Google Maps?

[reezekeys]

Here's a quick & small video of what I see in some – not all – Google Maps windows in Safari (9.1) on my MBP (Mavericks). Anyone know what it signifies? I have done "the google" and found zip – although I might be entering the wrong search terms (I tried "blue line bar OSX Safari google maps" in various combinations – mostly leads to iOS related stuff). TIA!

https://www.youtube.com/watch?v=6GNd_N3PEa4&feature=youtu.be