Further detail:
The composer window is locked open, but pressing discard will remove it. I can toggle the display with the thread button (bottom right corner) but I cannot submit the reply (probably because it was already submitted).
If I click the reply buttons in the thread while the composer is open, it does not clear the existing composer window and the submit button is still unavailable.
If I quote an existing reply, it updates the text of the composer with the quoted post, but submit button is still unavailable.
Pressing discard results in the [OK/Cancel] modal popup, and confirming on that popup does clear the composer.