Learned to make this time:
* tiled map editor with online co-op mode
* replays with reproducible physics simulation
* some platformer character state management

What could be done better:
* build and test tools ahead of time
* only use fully built and tested tools
* do not make a platformer ¦3

Forgot to mention! The coolest part is that every tile of solid ground is automatically clustered with others to create a solid body

It is represented by (very inaccurately placed) triangles with the same color for each solid body ¦3

Seamless surface removes a lot of jankiness from the simulation

And now it can interact with physics in real time!

Experimenting with multiplayer in-game tiled map editor ¦3

Not sure how useful the "multiplayer" part will be, but having it embedded into the game should be great for tweaking the level right on the spot