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I would be even MORE in love if that '80' columns was settable by either the review
author or by reviewer. In the latter case, I guess it would be just a local setting.

Would also be very nice if there was a default and per-user 'force wrap' setting at
which point the diff-viewer would wrap the line- ie I'd prefer to have an inserted
forced wrap rather than having to scroll the browser window.

(No comment was entered for this change.)

Another idea is to cut off lines longer than some length (say 90) with an ellipsis. 
Then show a simple tooltip for the rest of the line when the user hovers over the '...'.

Looking at wrapped code can be a pain too.

Maybe the background color could be different for characters after the boundary.
Example:
<div class="line"><span class="first-80">first 80 chars...</span>trailing chars...</div>
<div class="line"><span class="first-80">only 50 chars here...(30 chars
spaces)</span></div>

div.line {background-color: blue}
div.line .first-80 {background-color: white}