ai-app/docs/SCROLL.md

Scrolling in iris

This is the current scrolling design; iris/TODO holds open work.

Read this before touching iris/src/widget/position/scrollable.rs, scroll_area.rs, lazy_span.rs, or anything that pans, flings or lays out a long list.

The one rule

Everything a scroll position is made of lives in one ScrollController, and the widget that scrolls owns one. The position, the pending delta, the travel left, the pin, the DragGesture and the Flinger are all in that struct (scrollable.rs); there is exactly one Flinger and one DragGesture implementation in the crate's widgets. A widget with one implements Scrollable, whose one required pair of methods hands the controller back, and gets scroll, fling, drag, amt, is_scrolling, tick_fling and the pin as default methods.

Two widgets have one, and they differ only in how they spend a delta:

• ScrollArea (scroll_area.rs) — a fixed child, drawn and then slid about as a lump, which is what makes a scroll tick an O(1) move of one subtree. .scrollable(axis, pin) wraps anything in one.
• LazySpan (lazy_span.rs) — lays its own rows out from an anchor, so it cannot be a lump and is not wrapped in anything. Its own .scrollable() registers the same two senses against the controller it already has.

Do not give a widget its own fling, scroll amount, or platform wake handle, and do not add scrolling methods to the general Widget trait.

One convention for a delta

Positive scrolls the reader up or left; negative down or right. The content's pixels therefore move the positive way along the axis for a positive delta — the finger's direction — and that is Scroll::scroll's sign, Scroll::fling's, and Widget::apply_scroll's, from the gesture all the way down to a row's anchor.

It is a screen direction, not a logical content direction. A Dir::UP span's earlier content is below, so LazySpan::flip_delta converts public screen-space deltas into the walk's direction-relative space.

The contract between a controller and its owner

Two calls, both inside the owner's draw, because a draw is the only place that knows where the content ends:

1. take_delta() — everything a wheel, a drag or a fling asked for since the last layout, in one number, already clamped to the travel that layout reported. Clipping it stops a fling.
2. set_travel(Travel) at the end, plus whichever of moved_by (movement) or set_amt (an absolute position) fits how that owner knows where it ended up.

Travel is { back, fwd } in the same screen-space units as a delta: back bounds a positive one, fwd a negative one, and f32::INFINITY means "the end is not in sight". That last is not a placeholder — a lazy layout genuinely cannot say how far its content runs without walking there, and clamp takes the answer with no branch of its own.

Why the delta is banked rather than applied where it arrives. A wheel event, a drag frame and a fling tick all land between draws, and none of them can know whether there is content to move into. Applying them at the layout that follows is also what keeps layout a pure function of the state (Iris, 2026-09-08). The visible consequence, and the thing that catches a test out: amt does not move until the next draw.

Which clock a fling is ticked on. The vsync the frame callback carries, not Instant::now() -- on Android do_frame's frame_time_nanos, converted through the view's one DeviceClock (sense.rs), which also dates every touch sample, so a fling is advanced on the clock its own velocity was measured on. Frames are presented on an even cadence whatever clock they are computed on, so sampling the spline at "whenever the callback got to run" moves the content unevenly between frames that are shown evenly -- a shimmer that no frame-time percentile can see, since no frame was late. docs/RUST.md records the measurements.

Why a remainder was not enough

The apply_scroll(&mut delta) this replaced left the part it could not take in the caller's variable, and that 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. Now the owner reports what it did (moved_by, from the one place its anchor moves) as well as what it can do, and amt_counts_only_what_the_child_could_take is the test.

What amt means

The same direction for both owners, and a different origin:

• ScrollArea: distance from the start of the content, clamped into the scroll range. An absolute position.
• LazySpan: movement, not position. Paging rows in above moves the origin and the span cannot say by how much, never having measured them.

A scrollbar needs a real content length before it can use either, and a lazy span has none. Do not invent one.

ScrollArea::draw — draw, then place

1. take_delta, and move to where it asks.
2. Offer the child last frame's length and read the size it reports. An unchanged child returns from draw_inner without running draw.
3. Apply the pin and clamp against that size.
4. Place the retained drawing at its exact length and position. It is redrawn only if its reported size does not fit that box.

There is no measurement mode and no discarded drawing. Placing against the old length and letting the next frame correct it is not valid: layout must finish from the current state even if no later frame arrives.

The pin only re-pins on a frame with no delta of its own: the pin means "stay flush with the end as the content grows", and a reader who has just scrolled away has said otherwise.

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, and it drives its own controller: the walk is the only thing that can say how far it may go, so nothing above it is in a position to.

Why it is not a Span inside a ScrollArea

• A Span is skipped entirely in the steady state. When redrawn, it uses exact hints first, draws unknown fixed children forward from the cursor, and places retained drawings after flexible allocation. A child is redrawn only when its final box changes size.
• A ScrollArea'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 one would never update which rows it shows. That is why a LazySpan owns its controller instead of being wrapped in one.
• A lazy child cannot report a content length, so an area's clamp, end-pin and any future scrollbar would have nothing to work against. Walls are discovered by the walk instead.

Direction and pin are separate questions

LazySpan::new(dir, pin), and ScrollArea::new(inner, axis, pin).

• dir means what it means in Span: which end of the box item 0 sits at, and which way the sequence grows.
• pin is which end the view clings to as rows arrive.

A transcript is Dir::DOWN (oldest message is item 0, at the top) with Pin::End (the view sits at the bottom). Conflating the two would stand it on its head.

Start/End are content-relative; Neg/Pos are axis-absolute. They diverge for a reversed LazySpan. A scrollable acts on pinned_to_end, with dir resolving the chosen Pin.

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 render in two places at once, 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, and why it happens at all

Because the span cannot see the wall until it has walked to it. With rows loaded past an edge it reports INFINITY of travel that way, takes the whole delta, and the walk that follows discovers the content ran out 200px ago. Nothing else could be reported: the rows past the edge have never been measured, and measuring them is exactly the work virtualisation exists to skip. The other source is the content or the viewport changing under a settled anchor — a row that grew, a page dropped, the keyboard opening — where nothing scrolled at all.

So overscroll_gap measures the gap from the ends the walk already placed, and draw moves the anchor by it and walks a second time inside the same frame. moved_by counts that correction along with the move that caused it, which is why amt stays equal to what is on screen rather than drifting by every overshoot.

Layout is a pure function of state, not of how many frames have been drawn. 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.

Directional input

A scrollable registers CursorSense::drag(axis) and CursorSense::Scroll(axis). Horizontal and vertical gestures are distinct input semantics, so a higher horizontal row does not consume an undecided press that may belong to the lower vertical transcript. Both may observe the press start; after movement crosses DRAG_SLOP, the pointer locks to its dominant axis, only the matching listener receives the drag, and capture cancels every other listener that had been tracking the press.

Visual layers still decide priority between listeners for the same semantic. drag_senses() remains the deliberately direction-agnostic form for widgets such as selection that arbitrate the gesture themselves. Wheel input follows the same axis split; the desktop backend's Shift+wheel mapping produces a horizontal delta before dispatch, so it reaches the horizontal listener without a scroll-widget special case.

The transcript's wiring

app/src/ui/mod.rs, build_tree.

The transcript registers the wheel by hand rather than calling LazySpan::scrollable(), and this is not an oversight. That helper also registers a finger drag driving the span's own DragGesture, and the transcript already has an arbiter — SelectionController, 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.

SelectionController is attached directly to the span and holds its weak handle, then hands committed pans and releases through Scrollable::scroll/fling. There is no wrapper widget: TranscriptScreen::list is the layout (extent, key_at, jump_to_end) and the position (amt, fling, is_scrolling).

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, for 20, 200 or 2,000 total rows: 1 real draw and 1 child-coordinate move-slot write, no primitive rewrites and no text reshaped. The visible-row walk remains: it is what admits and retires rows at the viewport boundary, and a newly admitted row has real initial-placement work of its own. An idle frame is (0, 0, 0, 0) — draw_inner does not even enter the widget.
• A fully hinted Span draws each child once. Unknown fixed children draw provisionally and move; region-dependent children redraw if their final box has a different size.
• Moving the currently retained run as a unit does not require the lazy span's unknowable total content length. Its anchor supplies the relation between stable local row boxes and their desired screen boxes; one retained child-coordinate slot carries that translation. The offset is occasionally rebased after 65,536 pixels to preserve f32 precision, a rare O(visible) move-slot pass rather than steady-state work.

Verification

The unit and headless integration tests exercise direction, both walls, reversed hit-testing, fling registration, cancellation, nested horizontal pans, and transcript selection. docs/RUST.md defines the three test layers; use the cheapest layer that can observe the behavior under test.