You can test this problem using this QuirksMode page. The page notes other limits on Opera and the Macintosh version of IE, but on the current (v11) version of Opera, I was unable to duplicate it. I also don't have a Mac, so I couldn't verify it there, either. Only Firefox fails to copy the whole text block.
Apparently, this has been fixed in Firefox 4, but that's small consolation when you're tasked with supporting what people are actually using.