Prevent screen-readers from reading encoded raw MD

[adam-p]

A user has reported that the NVDA screen reader on Windows with Thunderbird reads out the encoded raw MD that's supported to be in an invisible <div> at the bottom of the message. I have confirmed in testing.

With some testing and research, I think I have found a fix. I'll be committing it soon.

[adam-p]

Fix committed in rev 9398820.

The primary fix target was Windows+Thunderbird+NVDA. Test results are as follows:

• Windows: Testing was attempted with Thunderbird, Outlook 2013 (h/t Tom Maynard), Windows 8.1 Mail, Gmail standard web interface, Gmail plain HTML web interface (Chrome and Firefox for web stuff). Results:
  • NVDA: Reproduced problem in Thunderbird, Gmail standard and plain HTML interfaces. Fix works well.
  • Window-Eyes: Thunderbird was broken before change, fixed after.
  • Thunder Storm: Could only get it to read Gmail in standard web browser interface. Didn't read MDH-encoded stuff before or after change.
  • JAWS: Tested, but couldn't get it to read email at all.
• OS X: Testing attempted using built-in VoiceOver feature. Apps tested: Thunderbird, Gmail standard and plain HTML interfaces (Firefox, Chrome, Safari), Apple Mail. Apple Mail was the only client where I was able to get email to read at all, and wasn't able to reproduce the problem.
• Android: Used TalkBack screen reader. Unable to reproduce problem in Gmail app.