In the Teams desktop client, you can pop out a chat or an app into a separate window to help you work on several things at once.
Pop out an app:
1. Go to the app you wish to open, whether it's an app pinned on the left of Teams or found under ellipsis. Right-click on the app icon, then select Pop out app
Pop out a chat:
1. On the left side of Teams, select Chat to open chat list
2. Find the chat you want, you hover over the chat and click the pop-out chat icon
Or select ellipsis, Pop out chat
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