# Scrolling in iris How anything in iris scrolls, as of 2026-09-08. This is the current design, not a history — `docs/IRIS.md`'s dated entries have the account of how it got here, and `docs/IRIS_TODO.md` has what is still open. Read this before touching `iris/src/widget/position/scroll.rs`, `iris/src/widget/position/lazy_span.rs`, or anything that pans, flings or lays out a long list. ## The one rule **`Scroll` owns the position, the gesture and the fling. Nothing else does.** A widget inside it either gets moved by it or is handed deltas to apply itself, and either way the scroll state lives in the `Scroll`. There is exactly one `Flinger` in the crate's widgets and one `DragGesture` implementation, both in `iris/src/sense.rs`. So: `.scrollable_on(axis)` / `.scrollable_to_end(axis)` is how anything becomes scrollable, including a `LazySpan`. Do not give a widget its own fling, its own scroll amount, or a `RequestRedraw` handle — that is what was just removed. ## One convention for a delta **Positive moves the content the positive way along the axis — the finger's direction — which brings *earlier* content into view.** That is `Scroll::scroll`'s sign, `Scroll::fling`'s, and `Widget::apply_scroll`'s. It holds from the gesture all the way down to a row's anchor. `LazySpan`'s internal `scroll` runs the other way (its anchor offset says where the pinned edge *sits*), and it is private, with the single negation inside its `apply_scroll`. There used to be two public conventions under the same name, and every call site had to know which widget it was talking to. If you add a third scrolling thing, it takes the finger's. `a_negative_delta_moves_toward_ the_end` (in `lazy_span.rs`) pins this across the whole handoff, because nothing else can catch a list scrolling backwards. ## The `Widget` handoff Three default methods on `Widget` (`iris/core/src/widget/mod.rs`): ```rust fn scrolls_itself(&self) -> bool { false } fn apply_scroll(&mut self, delta: &mut f32) {} fn scroll_offset(&self) -> f32 { 0.0 } ``` - **`scrolls_itself`** — "I position my own content; hand me deltas rather than sliding me about." Default `false`: an ordinary child is a lump its parent moves, which is what makes a scroll tick an O(1) move of one subtree instead of a redraw. - **`apply_scroll`** — take as much of `delta` as you can actually move, leave the rest. What comes back short is how the parent learns the content ran out. - **`scroll_offset`** — accumulated content movement, so the parent can keep an honest account. See "why the remainder is not enough" below. **`scrolls_itself` and `scroll_offset` are `&self` on purpose.** Reaching a widget through `Widgets::get_dyn_mut` *marks it dirty* (`iris/core/src/widget/widgets.rs`). Asking either question through a `&mut` path would dirty every ordinary child on every scroll tick and cost exactly the O(1) move the scheme exists for. `Painter::scrolls_itself` and `Painter::scroll_offset` go through `get_dyn`; only `Painter::apply_scroll` takes `&mut`. ### Why the remainder is not enough on its own `apply_scroll` leaving a remainder was meant to be the whole story. It is not, because **a lazy layout usually cannot say where its content ends until it has walked there.** With the wall out of view it takes the delta in full, and the walk that follows gives part of it back. So the remainder is exact only when the wall was already visible, and a parent adding remainders up would over-count by every overshoot and never correct. `Scroll` therefore reads `scroll_offset` *after* the placing draw and sets `amt` from it. `amt_counts_only_what_the_child_could_take` is the test; it fails if you try to go back to remainders alone. ## `Scroll::draw` — two paths `Scroll` reads `scrolls_itself` every draw and branches once, on the capability rather than on any concrete type. **Ordinary child (`draw_moved_child`)** — unchanged from before: offer the child a box as long as last frame's content length to measure it, apply the end-pin and the clamp against the measured length, then place it at the length just measured. Two draws, the second free unless the content changed. `amt` is a distance from the start of the content. **Self-positioning child (`draw_self_scrolling_child`)** — measure, apply, place: 1. `painter.widget_within(child, UiRegion::FULL)` — the measuring draw. 2. `painter.apply_scroll(child, &mut delta)` — the child takes what it can. 3. `painter.widget_within(child, UiRegion::FULL)` — the placing draw. 4. `amt = -painter.scroll_offset(child)`. Two properties make this work, and both are easy to break: - **The measuring draw is free in the common case.** It offers the same box as last frame, so with nothing dirty `draw_inner` returns immediately and the child's stored walls from its last walk are still correct — because nothing changed. When the content *did* change the child is dirty, really walks, and the walls are fresh, which is exactly when they need to be. - **Nothing is marked dirty by hand.** Reaching the child through `get_dyn_mut` in step 2 is itself what dirties it, so step 3 really draws rather than taking `draw_inner`'s unchanged-region skip. This is why there is no `Painter::draw_again` and why one should not come back: a mechanism for "give me a corrective frame later" is the thing this shape replaces. ### What `amt` means - Ordinary child: distance from the start of the content, clamped into the scroll range. An absolute position. - Self-positioning child: **movement, not position.** Paging rows in above moves the origin and the child cannot say by how much, never having measured them. The direction is the same as an ordinary child's; the absolute value is not comparable between the two. A scrollbar needs a real content length before it can use either, and a lazy child has none. Do not invent one. ## `LazySpan` `iris/src/widget/position/lazy_span.rs`. A virtualised sequence of variable-height rows, laid out from an anchor. It is what `Span` is, done lazily. It **does not scroll** — it lays out and answers honestly about how far it can go. ### Why it is not `Span::scrollable()` Measured 2026-09-08, and worth not re-deriving: - A `Span` is skipped entirely in the steady state, but **when it is redrawn it costs two draws per child** (21 draws for 10 children): phase 1 offers each child the ambient region to learn its length, phase 2 offers it its real share. So any mutation of a `Span` redraws all of it — 24 draws for 11 children after one prepend. - A `Scroll`'s efficiency and virtualisation pull opposite ways: a scroll tick offers a same-size moved region, `draw_inner` takes the `mov` path, and the child's `draw` never runs. A virtualising child inside a plain `Scroll` would never update which rows it shows. That is what `scrolls_itself` resolves. - A lazy child cannot report a content length, so `Scroll`'s clamp, end-pin and any future scrollbar have nothing to work against. Walls are *discovered* by the walk instead. ### Direction and pin are separate questions `LazySpan::new(dir, at_end)`. - **`dir`** means what it means in `Span`: which end of the box item 0 sits at, and which way the sequence grows. - **`at_end`** is the pin: which end the view clings to as rows arrive. A transcript is `Dir::DOWN` (oldest message is item 0, at the top) with the pin at the end (the view sits at the bottom). Conflating the two would stand it on its head. `Scroll::new`'s third argument is the same flag for the ordinary case. ### Two coordinate spaces, two conversion points The walk works entirely in **direction-relative** pixels from the leading edge — which for `Sign::Neg` is the bottom or the right. `Edge`, `Placement`, `RowExtent`'s `lead`/`trail` and every local are in that space, so the layout is written once for both directions. Exactly two functions know which way round the box is: - **`abs_region`** flips the box for `Sign::Neg`. - **`flip_pos`** converts the screen-space positions the public helpers speak in (`note_tap`, `key_at`, `extent`, all fed by pointer events). Skip the second and a reversed span hit-tests at the mirror of where it drew — which looks like a working list until you tap one. `a_dir_up_span_grows_upward_from_item_zero` guards this, and it asserts on where rows were **actually drawn** (`UiRenderState::active`) rather than on `extents`, because an `extents`-only assertion passes with the flip deleted: it checks the bookkeeping against itself. ### The row-height cache stays in the container `heights`, keyed by `RowKey`. Two reasons it cannot move into the framework: 1. **`ActiveData::size` dies exactly when it is needed.** The moment `LazySpan` culls a row it stops offering it a region, `draw_inner`'s old-children diff calls `remove_rec`, and the `ActiveData` — with its `size` — is freed. The framework's copy is gone for precisely the rows the walk has to pass through without drawing. 2. **A widget may one day render in two places at once** (Iris, 2026-09-08), so anything keyed by `WidgetId` alone that describes where or how big a widget was drawn will be wrong then. Where and how big belongs to the owner that placed it. Virtualisation *means* traversing rows without drawing them, and a size you can only get by drawing is no use for deciding not to draw. ### Overscroll `apply_scroll` only takes what the walk says is there, so **scrolling cannot enter overscroll.** What it cannot prevent is the content or the viewport changing under a settled anchor, and for that `overscroll_gap` measures the gap from the ends the walk already placed and `draw` moves the anchor and walks a **second time inside the same frame**. Layout is a pure function of the state, not of how many frames have been drawn (Iris, 2026-09-08). A correction that lands next frame is a frame drawn wrong, and there may be no next frame — a fling that stopped is not asking for one. ## The transcript's wiring `iris/transcript-ui/src/lib.rs`, `build_tree`. The transcript builds its `Scroll` **by hand rather than through `.scrollable_to_end()`**, and this is not an oversight. That helper registers a finger drag driving `Scroll`'s own `DragGesture`, and the transcript already has an arbiter — `Selection`, which must decide between panning and selecting text and so cannot let a second `DragGesture` see the same frames. `DragGesture`'s doc states the rule: one gesture, one arbiter, each frame delivered exactly once. The wheel handler registered here is identical to the helper's; only the drag differs. `Selection` is given the scroll area by `set_scroll_area` after the `Scroll` exists (rows need a `Selection`, and the `Scroll` needs the rows), and hands it committed pans and releases. `TranscriptScreen` exposes both `list` (layout, `extent`/`key_at`/`jump_to_end`) and `scroll` (position, fling, `amt`). A `Selection` with no scroll area still selects and still reports taps but cannot pan; there is a `debug_assert` in `drag` naming that. ## Measurements worth not re-taking - A settled scroll tick of a `LazySpan` with 31 rows on screen: **1 real draw and 31 move-slot writes**, no primitive rewrites and no text reshaped. An idle frame is `(0, 0, 0, 0)` — `draw_inner` does not even enter the widget. This is the number any "store the edges and only recompute what changed" optimisation would have to beat, and it is why the walk was left alone. - `Span`, redrawn: two draws per child (see above). - The one design that would collapse those 31 moves into a single delta write is moving the content as a unit, which needs a content length — which a lazy layout cannot supply. ## Still open **The pin lives in each widget, not in `Scroll`.** Iris asked for `amt` and the at-end control both to live in `Scroll` so a caller always edits the `Scroll`. `amt` does; the pin does not, because applying a pin happens when a row is appended — between frames, with no painter in hand — so it cannot arrive through `apply_scroll` as it stands. Moving it needs either a fourth `Widget` method or a parameter on `apply_scroll` (`apply_scroll(&mut self, delta: &mut f32, pinned_to_end: bool)` reads best: one method, and the signature says "here is the state you need from me"). Nothing external edits a pin today — the transcript sets it once at construction and calls `jump_to_end` on the span for the rest — so this is a design question rather than a missing capability. **Ask Iris which she wants before building it.** ## Tests that pin the behaviour In `lazy_span.rs`, all of these fail if the corresponding piece is undone: - `a_negative_delta_moves_toward_the_end` — the sign, end to end. - `amt_counts_only_what_the_child_could_take` — `scroll_offset`'s reason for existing. - `a_fling_stops_at_the_first_row`, `scrolling_past_the_start_lands_on_it_in_the_same_frame` — the walls, with no settling frame drawn on purpose. - `a_dir_up_span_grows_upward_from_item_zero`, `a_reversed_span_hit_tests_in_screen_space` — the two conversions. - `a_registered_fling_is_driven_by_tick_animations_and_then_unregisters` — a fling that nothing registers never moves, whatever its velocity. In `iris/transcript-fixture/tests/` (layer 1, no window or GPU): - `top_edge.rs`'s `scrolling_past_the_first_row_settles_on_it` / `scrolling_past_the_last_row_settles_on_it` — both ends, no settling frame. - `phone_screen.rs`'s `a_recorded_flick_releases_with_a_velocity_and_ flings_the_list` — the velocity against `benches/velocity_reference.py`'s number, and the fling's travel against `benches/fling_spline_reference.py`'s. - `phone_screen.rs`'s `a_long_press_and_drag_selects_text` — what caught two `DragGesture`s fighting over the transcript. - `catch_a_fling.rs`, `gesture_cancel.rs`, `fence_fling.rs` — press-catches a coasting area, cancels, and a code fence panning sideways independently of the transcript. `docs/RUST.md`'s "Three test layers" says which layer answers what. Test at the cheapest one that can answer the question; the emulator is for JNI, the IME, insets and one verification run, not for iterating on layout.