VSTS #1660777

Feature Request

User story:

1. Chat as normal
2. The end-user scroll somewhere up
   • The new message indicator should not show up
3. The bot/agent send a new message
   • The new message indicator will show up
4. Clicking on the new message indicator, or scrolling to bottom, will dismiss the indicator
   • The indicator only show up when the user is not at the bottom and a new message come in

[Enhancement]