I am in 
with the new full screen composer mode.
However, I do not understand why it is missing the “Post button”. If I am done editing, I need to first exit the full screen mode in order to post it. Note that the status bar with “saved” status at the bottom is there anyway, so why not expose the button as well?
The reasoning for this decision was already explained in the middle of this post.
No, you don’t. Press Ctrl+Enter.
Hovering your mouse pointer (if on a mouse capable device) over the reply button offers this hint as well:
Can we revisit this? I had a user get stuck today because of this and I only figured out what was going on after he sent me a screenshot. The original post explaining the thinking behind this decision is gone, but this seems like terrible ux to me to remove the button in fullscreen. The average user is not going to know to use Ctrl+Enter. I guess I can hide the full screen button…
Have a look here