Afloat is always on top solution on OSX, you can follow this post to understand what does it do.
Always on top in MacOS Yosemite
curl https://raw.githubusercontent.com/rwu823/afloat/sierra/install.sh | sh
open your terminal then copy and paste below:
curl https://raw.githubusercontent.com/rwu823/afloat/master/install.sh | sh
input your password ●●●●●●
Important !! Afloat doesn't support each app, the related discuss here #7 and #15.
Congratulation!! look at the Window menu.
Work fine on my OSX 10.10.x and 10.11, if any issues please post it.