oh man iscroll makes it so easy. `mouseWheel: true`

it has become a really good general-purpose abstraction layer for
handling touch / mouse / keyboard. excited to push the limits of the
architecture. nesting is hard but powerful. (pressure-sensitivity gonna
be huge.)

still need an abstraction layer for stepping/scrolling through general
state-space, e.g. for narratives. preferably branching. mathbox does
this well, particularly decoupling narrative/semantic state and view
state; i should try it out just for the sake of immersion in that mode
of thinking.