Steps 2 and 3 of the plan in docs/IRIS_TODO.md, together because
deleting the fling before `Scroll` could drive it would leave the app
unable to scroll at all. IRIS.md has the account and the measurements.
`LazySpan` loses its `Flinger`, its `density`, its
`Arc<dyn RequestRedraw>` -- which had no business existing in a
single-threaded frame loop -- its `tick`, and the whole
`fling`/`cancel_fling`/`tick_fling`/`is_scrolling`/`fling_velocity`
surface. `Scroll` was the only other `Flinger` user, so there is now one
implementation of the physics rather than two, and a transcript is
`list.scrollable_to_end()` like anything else.
Three new `Widget` methods carry the handoff:
fn scrolls_itself(&self) -> bool { false }
fn apply_scroll(&mut self, delta: &mut f32) {}
fn scroll_offset(&self) -> f32 { 0.0 }
`Scroll` asks the first, and a child that says yes is handed deltas
instead of being slid about as a lump -- which a lazy layout cannot be,
since which rows exist at all is a function of where it is scrolled to,
and it has no content length to be clamped against. `scrolls_itself` is
`&self` deliberately: `Widgets::get_dyn_mut` marks a widget dirty, so
asking through `apply_scroll` would dirty every ordinary child on every
tick and cost exactly the O(1) move the scheme exists for.
`Scroll::draw` is measure, apply, place -- the idiom it already used for
its own content length. The measuring draw is free in the common case
(unchanged region, nothing dirty, `draw_inner` returns immediately and
the child's stored walls are still correct) and really walks exactly
when the content changed. Nothing is marked by hand: reaching the child
to hand it the delta is what dirties it, which is why `draw_again` could
stay deleted.
`scroll_offset` was not in the plan and is needed. A lazy span usually
cannot say where its content ends until it has walked there, so it takes
a delta in full whenever the wall is not already in view and the walk
gives part of it back; the remainder is exact only when the wall was
already visible, and `Scroll` adding remainders up would over-count by
every overshoot and never correct. It reads the child's accumulated
movement after the placing draw instead, so `amt` equals what is on
screen. `amt_counts_only_what_the_child_could_take` is the test.
One convention for a scroll delta, the finger's. `Scroll::scroll(+)`
moved toward the start while `LazySpan::scroll(+)` moved toward the end,
with the latter's doc claiming to mirror the former -- so every call site
had to know which it was talking to. `LazySpan::scroll` is private now
and the single negation is inside its `apply_scroll`; call sites that
passed `-dy`/`-v` pass them through, and `phone_screen.rs`'s recorded
velocity flips sign with its magnitude unchanged.
`a_negative_delta_moves_toward_the_end` pins the sign across the whole
handoff, since nothing else can catch a list scrolling backwards.
The transcript builds its `Scroll` by hand rather than through
`.scrollable_to_end()`: that helper registers a finger drag, and
`Selection` is already the arbiter for those frames -- two `DragGesture`s
seeing one gesture is what its own doc rules out. Caught by
`a_long_press_and_drag_selects_text`, which failed when both were live.
Deferred, in DECISIONS.md and IRIS_TODO.md: the *pin* is still each
widget's own. Applying one happens when a row is appended, between
frames with no painter in hand, so moving it to `Scroll` needs a fourth
`Widget` method or a parameter on `apply_scroll`; nothing external edits
a pin today.
Verified: cargo fmt --check, clippy --workspace --all-targets clean,
cargo test --workspace green (21 suites), the arm64 release APK builds,
and the phone-shaped headless window replaying flick-120hz.touch scrolls
back through the transcript in the direction it did before.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
irisandClaude Opus 5
b7474f61b0