I agree it's kind of odd that our dev process is essentially a sea of broken states with intermittent workingness (if you count by states, I mean). Version control commits are an attempt to hilight/preserve those working states.

Limiting deltas to working-only states would, I think, result in something akin to MIT's 'Scratch' language, where kids put together puzzle pieces - a program can only be wrong at a semantic level, never at a source code level. OTOH, refactoring power tools available in modern IDEs are a bit of a counter-argument in that they allow code transforms that don't usually make much of a semantic difference (eg. 'inline this', 'extract this as a variable', etc) But as you say, that's "written code" not "writing code".

For writing code, I think the closest current IDE feature to tylr would maybe be syntax hilighting. It'd be cool to maybe have some kind of 'expression hilighting' mode, and maybe an 'expression editor' mode with actions like tylr supports.