I just use the desktop site - not really a fan of mobile anything. Didn't even do reddit on mobile. Hopefully what you're describing will roll out to more instances sooner rather than later though.

Edit: I do actually have a button to expand text posts - it's a small square that appears on the right side of titles. Not sure if it's brand new or if I just never noticed it until now. Cool.

Edit again: Youtube videos also can open in-line now, plus some videos from other places. Awesome. Guessing it's related to this update