Alastair reported on the Orca list:

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On the page http://www.gentoo.org/doc/en/power-management-guide.xml do a search for "listing 2.5" and try and arrow down through the lines which follow.  I've found that it gets stuck at certain points and just keeps saying blank and not moving down.  I then need to arrow right to get to the next line.  I've had a brief look at the html and the only possible cause i can see is the use of the span tags, but i can't reproduce the problem in html myself.
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It's not the spans; it's the double line breaks. :-(

As part of last release's line navigation refactor we largely abandoned the use of find{Previous,Next}CaretInOrder() and checking extents. As a result, we had to be clever about whitespace -- in particular line break characters. We're not being clever enough. As a result, we have the above issue as well as bug 534431 (skipping over line breaks when composing certain messages).

Rather than try to anticipate every case (which is becoming increasingly difficult given changes in Firefox 3.1 seem to include exposing whitespace differently from 3.0), we should rely upon checking extents when verifying that we have located a new line.