I just noticed that when I replied to a blog post that if I go full screen, the text box extends beyond the bottom of the screen. I just noticed it does the same thing here. In order to see the bottom I have to go to 50% zoom. I hadn't noticed before since I never typed enough to fill the screen in fill screen mode. I usually just need to use fs when I fill the smaller box. I would then shrink it back down to use the 'Post' button, never realizing it was there in fs mode.
I am using Google Chrome Version 61.0.3163.100 (Official Build) (64-bit)