I see it's now possible to see the edit box and the preview at the same time, which is great. However, in this state the "Preview" !button is called "Keep editing" and its function is to hide the preview. That label doesn't make sense any more, since I can now keep editing without pressing the !button. The function is not too useful either, what I'm more likely to want to do is to update the preview after some editing, not to hide it, but that requires two clicks of this !button (and moving the mouse to find its new position in between).
Also: what's the rationale for hiding the "Cancel" !button while in preview mode? I now have to klick "Keep editing" to reach "Cancel", that's a bit like clicking "Start" to shut down the computer...
Is "Add topic" a good label for the submit button? That could be interpreted as "start yet another topic". The corresponding button on wikipages is called "Save page" (and looks like a button, yes we can!), so to be consistent I would suggest "Save" here.