If you are using Outlook on the Web (aka OWA or Outlook Web Access), you probably noticed when you could start/continue a Teams chat using the chat button available top right of OWA.
But it is launching another page to use Teams on the web, making you leaving OWA.
Well, good news as now you can start/continue a chat without leaving OWA.
NOTE deployment is under way and the experience may not be available for all of your users.
When hitting the chat button, you will see options to either continue the last 7 chats, start a new one or event launch the Teams web client.
If you start/continue a Teams chat, the chat pop-up will be displayed bottom right of your OWA.
You can have multiple chat pop-up by simply starting/continuing new chats.