The same problem happens on the iPhone. It appears to be a bug in the mobile version of Safari.
The vBulletin forum software specifies a fixed-height box with the overflow:auto property, which tells the browser to include a scollbar if the content doesn't fit, and no scrollbar otherwise. But no scrollbar shows up, so the bug seems to be on the browser side, not on the MacRumors side.
If you copy the code block and paste it into a text application, you'll see all of it, but that makes a lousy workaround. A better workaround is to reply to the post; you'll find the full content in the message area.