If you drag the little resize nub on the box down far enough, eventually the preview button stops working. It just makes it scroll down a little, but doesn't open the preview window.
It's been like this as far as I can remember, so about 6 years, two browsers - Chrome and Firefox, and two OS's - Win7 and Win10. I always got around it by making the box smaller before clicking preview.
@sciller45 It appears to be a bug in the jquery.markitup library that the forums use for their editor. I was able to reproduce the same issue when using server-side previews on their demo/sample site. That said, I think I was able to fix it by tweaking how the preview button is built. It works for me on Firefox and Chrome now.
