The browser test "Replying.Canceling reply leaves usable form" caught this. I think it's to do with the way Flow triggers progressive enhancement. Existing Reply buttons work, clicking in the [Reply to "topic title"] textarea works, and the problem goes away on reload; so this is low priority.
The Flow team tracks this in https://trello.com/c/pruorgkD
Version: master
Severity: minor
Given I have created a Flow topic with title "Reply watch test" And I start a reply with comment "my form lies over the ocean" And I click the Preview button # Create topic then click its Reply doesn't have a cancel button (bug 69412), so this fails. And I click the Cancel button and confirm the dialog And I start a reply with comment "bring back my form to me" Then I should see the topic reply form
Expected:
Cancel button should be visible and clickable and above test should pass.
The JavaScript bug was fixed some time ago.
https://gerrit.wikimedia.org/r/#/c/159009/ updates the test.