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
open your terminal then copy and paste below:
curl https://raw.githubusercontent.com/rwu823/afloat/master/install.sh | sh
input your password
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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.