This is similar to Windows 10 Mobile: Text editor falls behind virtual keyboard when typing but is windows 10 on a tablet (rather than mobile).
When in touch mode, when attempting to reply to a post or create a new post, the bottom panel gets hidden by the virtual keyboard. This can be seen in both Firefox and Chrome (desktop versions). The behavious is different in Edge which does move the box to above the virtual keyboard (but the page rendering is less good).
They input box is there (honest).
Edit - I have discovered you are not alone. The compose box on Gmail (even using Chrome) has the same issue. This can be got around in Gmail to some extent by expanding the compose window (box).