When adding a topic to an existing flow board on mobile (tested Chrome on Android 6, on a Nexus 5x) the 'Add topic' button is grayed out and unusable when following my standard linear workflow:

• go to a Talk: page in Flow mode (tested on office.m.wikimedia.org)
• tap into 'Start a new topic' area and type "Test"
• tap into 'Post a new message to "Talk:xxx"' text area and type "Test"
• scroll down or hide keyboard to reveal the "Add topic" button

Expected behavior:

• "Add topic" button should be tappable; tapping should save the comment

Actual behavior:

• "Add topic" button is grayed out/disabled; tapping does nothing

Workaround:

• Tap back into the topic area
• Type a letter
• Hit backspace
• Scroll back down to "Add topic" button, which is now active
• Tap "Add topic" button to save

Screenshot: