I have an idea about this one. I was able to reproduce it, and I was able to turn it off.

Have you at any time installed the Photoshop CS5 optional plug-ins?

http://kb2.adobe.com/cps/828/cpsid_82824.html

And if you don't know the answer to that question, then try this one:

When you go to the folder

/Applications/Adobe Photoshop CS5/Plug-ins/Extensions

are there any files that start with the word "Overscroll" ?

If you answer yes to either question, then one of the Overscroll plug-ins is the reason you are seeing this behavior. If you don't like what it does, you can remove them. The Overscroll plug-ins are installed with the optional plug-ins package.

That's because you're not seeing more canvas. You're seeing more "apron" around the canvas. The canvas size is the same as the image size in your document, and that's not changing. It's the area outside the canvas that's changing, and that's expected when the Overscroll plug-ins are installed.

Glad that worked out! Now, for some back story and other answers...

What is the point of these plug-ins? The only reason I recognized the issue is that I like the Overscroll feature. Here's the problem it solves: If you're drawing a path or transforming an object, and some path or transform handles end up outside the canvas, and you happen to be in a magnified view in a standard mode floating window, you cannot reach those handles. The scroll bars stop at the canvas edge. In a normal Photoshop installation, I have to stop and switch to one of the other modes (Application Frame or Full Screen) to be able to scroll or pan past the canvas edge. Or I have to zoom out, which means I can't do a detailed edit. With Overscroll you can get at those beyond-the-canvas handles at all times regardless of magnification level or window mode.

Seeing the screen shot scrolled past the canvas while zoomed in was what clued me in.

Why didn't the Adobe support rep figure it out? Because his script probably tells him to start from a fresh installation, which is missing the optional plug-ins, and unless he's heard of them and remembers them he won't be able to reproduce the behavior.

While I would prefer that Overscroll be the default, it looks like Adobe was correct to make it optional since the confusion it caused is why this thread got started. Everybody expects traditional OS windows to stop scrolling at the edge of the document. As far as new books, Mr. Lao, I'm not sure I'm doing any before the next Photoshop upgrade, but you'll find some web articles and other projects turning up between now and then.