Angelo Pesce, Roblox technical director, shared his live coding framework earlier. Looks a lot like a tool I've been playing with and started independently https://twitter.com/kenpex/status/1262235296483037184?s=19

A similar approach to mine too. I like the C interface though, that is interesting for sure. I have a similar shader list, though they are tabs in the editor. I have a scene description format too for mutipass rendering; but I've been thinking about scripting that for more flexibility. A C interface would be interesting....

It's very experimental and it's not clear from that picture what is special about the tiny 3d editor but for example there is http://threejs.org/editor as an experiment. You can put a function on any object. If you're used to JavaScript

this

is the object the function is on and

event

contains

time

and

deltaTime

I know it's far from complete. Just that it's fairly trivial to throw together a live coding editor at least to start. A hard part becomes handling all the loading and unloading of resources and null pointers etc. For example you make a script for player and it has a reference to a gun model in the scene graph and then a user selects the gun model and presses delete. You can build a system that handles that but it's not the default. A game engine without tools would generally just have some direct pointer to the gun model in the code controlling the gun. Yes, you can build up a system to handle that but suddenly your easy pointers because some kind of heavy handle. Or you can use a language that can hopefully handle it for you. AFAIK C/C++ is not one of them though. You've also probably seen https://playcanvas.com/ which could basically be generously called Unity Editor online.