Currently (May 2020) the chat window in Teams is not yet detachable, and has no way to be floated on top of other windows. This can create a challenge if you are using a powerpoint presentation and sharing this with a class during a meeting, as when you present it takes up full screen and hides the chat.
The fix for this is to change the slideshow mode for powerpoint. See the screenshots below:
Once you select this option, when pressing present the PPT only takes up the window space allocated to Powerpoint. You can now resize the Powerpoint and Teams apps so that the chat sits behind the powerpoint and is still visible:
You can not share your screen or the powerpoint with the meeting.