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>
irisandClaude Opus 5
bf8658c404