I agree it would need some specific support from the host REPL. There is a terminal emulator for Emacs, vterm, which I assume could provide that "terminal emulator widget" functionality. But you'd probably have to add code to SLY/SLIME/etc to support this. (To be honest I have never used SLY or SLIME, but as far as I know they don't have such a feature already.)
I think the other problem is that it is easy to launch a new buffer in Emacs hosting the terminal emulator. But what I was suggesting was more like embedding a buffer inside an existing one. I don't know if Emacs has that feature, but if it doesn't, again there is no reason why someone couldn't add it. (I don't know if Emacs Lisp is sufficient to implement such a feature or if extensions to the C code would be necessary.)
skissane|5 years ago
I think the other problem is that it is easy to launch a new buffer in Emacs hosting the terminal emulator. But what I was suggesting was more like embedding a buffer inside an existing one. I don't know if Emacs has that feature, but if it doesn't, again there is no reason why someone couldn't add it. (I don't know if Emacs Lisp is sufficient to implement such a feature or if extensions to the C code would be necessary.)