This is exciting news that PGOffline will be rewritten.*
A small request comes to mind:
When hovering the mouse over a link in the message body, sometimes you can see the URL you'll be taken to, and sometimes you can't since the writer of the message labeled the URL with a title.
Hovering the mouse pointer over the link should cause the "..." part of the href code to be displayed.
Possibly the status bar would be a good place to display the URL, as occurs in a web browser.
Thanks for considering this.
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* This must mean you are confident that Yahoo will be sticking to its formats and not pulling the rug out again...