I tried your suggested edits to get the Close button. A few things:
- Your edits are for a close link and it is in the wrong place. It should be a button and located to the right of the Reply button once the window pops open.
- So the Close link should be a button that would be better labeled as "Cancel" and located just to the right of the Rely button not the link at the bottom of the popup.
- The Close link should not show when there is nothing opened yet.

A Small Issue:
- Once you click "Close" if you decide to reply to that same message you can't. The window will not pop open a second time.
- After you click "Close" the only way to reopen the same message if you wanted to reply and changed your mind is to refresh the whole page. YUCK
- How can this be fixed?
Thanks for the edits. A broken close is better then no close.