actually it doesn't :)

The main point of the previous logic was to not remove the subject if the user keeps the backspace key pressed. He will need to actually press the backspace key once more once the subject is empty.

Now, this was not working properly because of a bug in the keyboard, and that's why you don't see a difference. The good news is that the keyboard has been fixed tuesday :) (bug https://bugzilla.mozilla.org/show_bug.cgi?id=960946) So you need a recent gecko + a recent master to actually see the intended behavior.

I guess you just need to get the previous behavior back :) It was not the goal of this patch anyway.