If you change the message format while composing, then visit the address book to get some addresses, then return to the Compose window, it will have reverted to the default formatting instead of the one you chose. This is especially unfortunate given that changing the format again at that point wipes all existing form input, so you can't use the address book on a message you want to change the format on.
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P.S. I've just realized I wasn't using the Javascript address book, and of course enabling that makes this problem go away, as the Compose window is never redrawn. But it's still a problem if you want to preserve the non-Javascript functionality.
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What do you mean exactly by "the message format"?
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By message format, I mean plaintext vs. HTML. Ie. set your default formatting to plaintext, open a Compose window, click the Compose in HTML button. Then go to the Addresses (without the Javascript option enabled), choose some and hit Close or just hit Cancel to return to the Compose window. It comes back in plaintext, not HTML. And because of the other bug, clicking back to HTML at that point erases all previous input.
This is with SquirrelMail 1.4.8, and Firefox 1.5.08 on XP.
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Ok, so this is the HTML Compose plugin, reassigning.