Ahh, maybe you're talking about having the Reply Page Thread View in a scrollable box, like in UBB or something, not the entire page.
Yeah, you'd have to make a Mod for this. And now that I think maybe I understand what you're asking, I actually wouldn't mind this feature myself.
It seems like it would be relatively easy to do, though... just have to figure out where the HTML begins and ends for the thread view on the reply page, and add those pesky <textarea> tags around it.

If anyone has any ideas regarding the following problem, though:
(In message post window:)
[shadow=red,left,300]TEXT[/shadow]"I want this text to be shadowed when I highlight it and hit the shadow button, instead of just putting a generic shadow tag with 'TEXT' in it!"
Then PLEASE let me know!