Thread title without comment enables the "create thread" button, but not really

[cellio]

meta:291583

If you start a comment thread, the "create thread" button starts out disabled. If you enter a comment of the minimum length, the button is enabled. If you don't enter a comment but do enter a thread title (which is optional), the button is enabled. This isn't harmful because clicking the button produces an error directing you back to the comment textbox, but ideally the button state would not be affected by the optional title at all. It should be based only on the comment length.

I'm looking at app/views/comments/_new_thread_modal.html.erb, but I can't quite figure out where the button display state is set.