The app seems to work for me after I've got some problems at first.
Not sure if enabling the picture-in-picture chrom flag was the solution, but somehow it works now.
So thank you for your great work on this one. Was looking for something like this on chrome for quite some time.
The only thing that's still bugging me a little bit is, when I click the chrome extension on Youtube a message dialog pops up with a long URL (shortened it):
betterpip://open?url=https%3A%2F%2Fr2---sn-4g5e6nls.googlevideo.com%2Fvideoplayb
ack%3Fmt%3D1522330741%26ratebypass%3Dyes%[...]
After clicking "OK" it opens the PiP view where I can watch the Video.
So, is it possible to somehow get rid of that first message so it directly opens the PiP video?