It’s a bug. And annoying one. I haven’t 100% figured out what triggers it,
My workaround is to close the dialog, quickly copy the post by clicking in the box and pressing Ctrl-A followed by Ctrl-C, and then refresh the page.
Usually, the post will come back when I refresh the page. It will either just pop up on its own, or it will be collapsed down to the bottom of the page like a draft, and I’ll be able to click the up arrow. But, on the off chance that it’s completely gone, I can just click Reply again and paste the contents.
If I do ever figure out exactly what I did to trigger it, I’ll report the bug.
BTW, do note that “new window” and “new tab” are considered synonymous as far as the web page is concerned–at least, in most circumstances.