This happened a couple of weeks ago and I almost started a thread but logged out and in again and it self resolved.
I can't even tell you what's wrong. It seems to be related to copying and pasting as that's what triggered it last time. I copied something to add to a reply and when I attempted to add it, the whole page changed to look like an entirely different thread I'd previously visited. Tapping on the screen and trying to scroll brought the message box back but for example, if I noticed an error in what I was typing and clicked on the error in order to delete/overwrite it, the same page would suddenly appear. I know that doesn't make sense. I've just copied and pasted something else and it's doing it again. For example, if I click on the word 'copied' in the sentence above as if I were going to delete it or just to move my cursor up there to add something, I'm suddenly transported to seeing an entirely different page (a thread in chat about Christmas in fact) but I'm unable to interact with that thread iyswim. It's just like a screenshot of it almost.