Positive scrolls the reader up or left and negative down or right,
whichever way the widget receiving it lays its content out (Iris,
2026-09-08: "that way it always works as the user would expect").
`LazySpan` took the delta straight into the direction-relative space its
walk works in, so a `Dir::UP` span -- whose later content is *above* --
panned the opposite way from every other scrollable in iris for the same
number. `flip_delta` is the conversion, the counterpart of the `flip_pos`
that positions already went through, and the two places that meet the
outside world (`apply_scroll` and `moved`) are the only ones that use it.
Nothing built a `Dir::UP` span yet, so this was latent; the existing sign
test could not have found it either, since it asserts in the walk's own
space where both halves agree with each other while disagreeing with the
screen. The new test compares the two `dir`s against where rows were
actually drawn.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
For the next session, since this one is about to be cleared. Current
design only -- `Scroll` owns the position, the gesture and the fling; a
child is either moved or answers `Widget::scrolls_itself` and is handed
deltas; one sign convention, the finger's. It carries the things that are
expensive to rediscover and easy to undo by accident: why the two `&self`
capability methods must not be `&mut` (`get_dyn_mut` marks dirty), why
`scroll_offset` exists beside `apply_scroll`'s remainder, why the
measuring draw is free, why nothing is marked by hand, why the height
cache stays in the container, why the transcript builds its `Scroll` by
hand instead of through `.scrollable_to_end()`, and the measured numbers
behind "a `LazySpan` is not a `Span`".
Also names the one thing still open -- the pin -- with the two ways to
close it and an instruction to ask Iris rather than guess.
`scroll.rs` and `lazy_span.rs` now point at it from their module docs
rather than restating it, AGENTS.md lists it beside the other design
documents, and IRIS_TODO.md's in-progress entry defers to it.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
irisandClaude Opus 5