Currently when we display a message, we convert it to HTML and display it in a browser. Then when we print the message, we convert it to different HTML and print it. We shouldn't do this, we should use the same HTML and have CSS create the different versions. This means that (a) we don't have to convert a message to HTML twice and (b) we can print the message displayed in the browser without having to jump through a lot of hoops. Converting to the new printing UI will be much easier, at the cost of cleaning out some horrible old back-end code and possibly causing regressions.

I've just realised that if I do this then extensions with message display scripts could get messed up, particularly if they do silly things like our tests do:

document.body.textContent += "Hey look, the script ran!";

I think this makes nsMimeOutput::nsMimeMessagePrintOutput obsolete, there are some uses left behind but they should behave as nsMimeOutput::nsMimeMessageBodyDisplay does.
I'm not even sure most of the remaining uses are in live code.

Sorry Julien, is this the bug you meant to add that to?

https://hg.mozilla.org/comm-central/rev/a029a4c93fb0

Hi Geoff, this bug blocks bug 1711408 so I flagged is as a Proton bug and have created the JIRA correspondingly. Hope it helps, let me know otherwise.

(In reply to Julien Chaupitre from comment #5)

Hi Geoff, this bug blocks bug 1711408 so I flagged is as a Proton bug and have created the JIRA correspondingly. Hope it helps, let me know otherwise.

Hi Julien, this bug is bug 1711408, so it cannot block itself. This bug is in MailNews Core: Printing (Thunderbird territory), so it's unlikely that this relates to Firefox Proton UI changes, and I've never seen such links on any other Thunderbird bug.