Weird. You're saying none of the other forums here give you the posting window except this one? I agree with John's initial test: log out, then back in.
Try another skin here, too - any of them. I've noticed that one or two of them act differently in what I can see. I'm sticking with TPF Red V1 for now, both on laptop and my phone.
Also, if needed, try the above with different browsers. Since this clearly hasn't happened before, and you've been a member since 2007, I'm thinking this is likely a hiccup that can be tinkered with and resolved by scrambling a few different things.
I'll be more than happy to report this thread so the TPF tech crew will see it, if none of the above works.