This is possible and a great idea in my view. In fact this is something Patchou should try and implement himself. However this would be full of limitations, and would look very different to the rest of Messenger (using Plus! could make it look nice mind you).
You could not reply within the window itself, and it would have to be a seprate window from your other conversations. Plus adding fancy effects such as the bubbles would be difficult (but not impossible), this would most likely not blend in with the general design trend of Messenger, and will look bad. Finally, easily distinguishing between contacts (using those nice display pictures) is impossible.
All these factors will make creating this feature extremely diffcult and full of limitations. If anything the Messenger team should code it themselves
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