When you click on a message's Reply button to reply to it, the original message you're replying to is located below your edit box.
In the upper right corner of the original message's title bar you will see two radio buttons (clicking on one to select it deselects the others): Normal: and Peek Mode:.
If you select Peek Mode:, then that changes the display of the original message to show all the embedded dBCodes. That is almost exactly like what happens when you click on the Peek button, except you stay in the same tab as your edit box.
The benefit of this is that when you copy-and-paste from the original message then you can also copy all the formatting codes, including url tags containing links to other sites.