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Thank you for reporting this to us Thom.
I am not able to recreate it, although I have re-styled my own reply, but that is a different thing :)
Now, the current reply box will resize as it is needed by utilizing JS code. Unfortunately, if you add a photo and do not make any changes, the height of the reply textarea will not resize itself.
Now, since you say that the Post button disappears, I presume that the issue that you have is actually a tad different - where the box resizes, but the button to post gets moved under some other element?
Could you please take a screenshot of the issue and let us know what browser you are using - just to make sure that it is not a browser issue?