New Post: Double clicking on attachment row gives a new window

I tried your steps as best as I could decipher them. I didn't see anything unusual. So either I saw your problem, but because I know what I'm looking at, didn't think it was a problem, or I didn't. Either way, I still don't know what the issue is.

You asked "Which window is this?". You never mentioned a window before asking this, so I have no idea what window you're talking about. That's why I thought a screenshot might help.