Howdy folks 
I originally wanted to weigh in here and join in the calls for this feature, but after diving in a bit deeper I wanted to share what I learned about how this works (in case anyone missed it like I did initially!)
We’ve just embedded Discourse as the comment system for our blog and I had a little mini freak out when I clicked the “Show full post…” button and saw the whole blog copied without the correct canonical URL 
After taking a few deep breaths I went into my “debug mode” and started checking the straight HTML response and checked how much of the post is actually there. As it turns out only the initial paragraph is included in the HTML and therefore this is all Google will see. Phew!
Having a second look at it, it makes perfect sense in the way the UX is laid out. I’m assuming the reason it’s hidden behind a button is because you want people to be able to read the full post and not affect SEO 
I guess initially I was surprised that that “Show full Post…” wasn’t just a link to the original blog 
but I guess it’s an OK way to do it 