Function ride 

fn ride(
    indexed: &IndexedDocument<'_>,
    path: &BlockPath,
    presentation: &Presentation,
    riding: Riding<'_>,
    sink: &mut CommitBuilder,
) -> Option<Rect>

Fold what the gesture has written so far onto a scratch prediction, re-solve against that document, and author every corner list the solve promotes. Trims, re-solves and promotions therefore land in the same commit as the gesture that caused them, and the “every corner is a waypoint” invariant survives without the solver ever writing.