Yes, surprisingly the OS X Messages app doesn't seem to share a lot of UI code with the iOS version. You can easily tell that it's a simple WebView from the way text selections behave.
Strangely Apple seem to build everything twice, once for iOS, once for OS X, even if they have the same appearance. It'll bite them some day.
Meaning how you can select text across messages?
Yes, that and how some of the whitespace between the messages gets selected as well.
Yes it surprised us too, we didn't even think about looking into Messages until Shubs jokingly started sending payloads to me using it.