You can't resize the text area by dragging the split between it and the main page? Or you can but just get empty space? I wonder if it's treating Opera 12 as a mobile browser or something.
I think they have been working on the text editor recently so it could be due to a recent change, and might be fixed in a later change. Or not... They may not be aware.
If you see the same thing on the main Discourse site, it might be worth reporting to them, although there's no guarantee they will still want to support an old browser. (Unless it's still important on Linux or something, which the old thread suggested was the case back then at least.)
If it's not the same on the main Discourse site, I'd wait to see if an update here fixes things again, or an update there breaks things later on in the same way. (I'm not sure how frequently their forum updates, although I'd guess it keeps more up to date than we do, and we update every few weeks, unless there's a reason to grab one more frequently.)