iris/src/layout_tests.rs

//! Pass conditions for LAYOUT.md section 8, exercised as plain unit tests
//! rather than through `run-headless.sh`: `UiRenderState` and `Widgets` do
//! not touch a GPU or a window, so a tree can be built and driven directly.
//! No GPU-backed rendering (`UiRenderNode`) is exercised here -- only the
//! CPU-side layout/move machinery LAYOUT.md is about.

use crate::prelude::*;

/// The minimal `UiRsc` a test needs: just the shared `UiData`, none of the
/// event/window/state plumbing `DefaultRsc` carries. `pub(crate)` so
/// `access_tests.rs` (I4, RUST.md) can reuse it rather than keeping a
/// second copy of the same harness.
pub(crate) struct TestRsc {
    pub(crate) ui: UiData,
}

impl UiRsc for TestRsc {
    fn ui(&self) -> &UiData {
        &self.ui
    }
    fn ui_mut(&mut self) -> &mut UiData {
        &mut self.ui
    }
}

/// A `ScrollArea` over a `Span` of `n` fixed-height rects -- N primitives large
/// enough that an O(N) regression in the move path would show up as a
/// non-trivial counter rather than being lost in noise (LAYOUT.md section
/// 8, condition 3, using rects rather than glyphs to avoid pulling the font
/// stack into a plain unit test). Returns the scroll widget (weak, for
/// mutating it later), the erased root to draw, and the rows (weak, for
/// hit-testing one of them).
fn scrolled_rects(
    rsc: &mut TestRsc,
    n: usize,
) -> (WeakWidget<ScrollArea>, StrongWidget, Vec<WeakWidget<Rect>>) {
    let mut span = Span::empty(Dir::DOWN);
    let mut rects = Vec::with_capacity(n);
    for _ in 0..n {
        let rect = rsc.ui.widgets.add_strong(Rect::new(UiColor::WHITE));
        rects.push(rect.weak());
        // Each row gets a fixed height so the span's total content is
        // genuinely taller than the viewport -- rest-sized rows would just
        // divide whatever space is offered and never need scrolling.
        let row = rsc.ui.widgets.add_strong(Sized {
            inner: rect.any(),
            x: None,
            y: Some(Len::abs(10.0)),
        });
        span.push(row.any());
    }
    let span = rsc.ui.widgets.add_strong(span);
    let scroll = rsc
        .ui
        .widgets
        // Anchored at the *start*: every test below scrolls down from
        // the top and states its sign convention against that. An
        // end-anchored area now sits at its end from its first drawn
        // frame (`Scroll::draw` measures and places in the same frame),
        // so `Pin::End` here would mean scrolling down from a
        // position that is already the bottom -- a clamped no-op, which
        // reads as "the move path is broken" rather than as the test
        // starting somewhere it did not mean to.
        .add_strong(ScrollArea::new(span.any(), Axis::Y, Pin::Start));
    let weak = scroll.weak();
    (weak, scroll.any(), rects)
}

#[test]
fn an_unchanged_frame_draws_and_rewrites_nothing() {
    let mut rsc = TestRsc {
        ui: UiData::default(),
    };
    let (_scroll, root, _rects) = scrolled_rects(&mut rsc, 500);
    let mut render = UiRenderState::new();
    render.resize((800.0, 20000.0));

    render.update(&root, &mut rsc);
    // Two, not one: the first offers `ScrollArea`'s content the container's
    // own length as a placeholder (nothing has been measured yet) and
    // `Scroll::draw` asks to be drawn again once it knows the real one,
    // which the second update is. Only after that is the tree settled --
    // see `scrolling_moves_in_o1_without_a_redraw`'s own note on the
    // same first draw.
    render.update(&root, &mut rsc);
    render.take_counters(); // discard the first, real draws

    render.update(&root, &mut rsc);
    let (draws, rewrites, moves, _shapes) = render.take_counters();
    assert_eq!((draws, rewrites, moves), (0, 0, 0));
}

#[test]
fn scrolling_moves_in_o1_without_a_redraw() {
    let mut rsc = TestRsc {
        ui: UiData::default(),
    };
    let (scroll, root, _rects) = scrolled_rects(&mut rsc, 500);
    let mut render = UiRenderState::new();
    render.resize((800.0, 600.0));

    // The first draw offers `ScrollArea`'s content a zero-height region
    // (nothing has been measured yet) and learns the real content length
    // from what comes back; `update()` only redraws widgets actually
    // marked dirty, so that corrected length is not reflected in the
    // content's own *active* region until something -- here a no-op
    // scroll tick -- actually asks `ScrollArea` to redraw again. Only after
    // that warm-up does the content's offered size stop changing between
    // draws, which is what makes a further, real scroll tick a same-size
    // move instead of a resize. See scroll.rs.
    render.update(&root, &mut rsc);
    rsc.ui.widgets.get_mut(&scroll).unwrap().scroll(0.0);
    render.update(&root, &mut rsc);
    render.take_counters();

    // Negative: `scroll`'s sign convention subtracts from `amt`, and
    // `amt` starts at (and is clamped to) 0 at the top of the content, so
    // a *positive* argument here would be scrolling further up (a no-op,
    // already clamped) rather than actually moving anything.
    rsc.ui.widgets.get_mut(&scroll).unwrap().scroll(-40.0);
    render.update(&root, &mut rsc);
    let (draws, _rewrites, moves, _shapes) = render.take_counters();

    // The pass condition (LAYOUT.md section 8, condition 3) is 0 draws and
    // 1 move_offsets write, independent of how many rects are in the
    // scrolled subtree. `draws` here is exactly 1: `ScrollArea` itself is
    // marked dirty by `scroll()` and its own body is cheap arithmetic with
    // no primitives of its own, so it is the one real `Widget::draw` this
    // counts -- the 500 rects underneath move via the O(1) chain and are
    // never revisited.
    assert_eq!(draws, 1, "only Scroll itself should redraw");
    assert_eq!(moves, 1, "the scrolled subtree should move in one write");
}

#[test]
fn hit_testing_follows_a_scrolled_widget() {
    let mut rsc = TestRsc {
        ui: UiData::default(),
    };
    let (scroll, root, rects) = scrolled_rects(&mut rsc, 500);
    let mut render = UiRenderState::new();
    render.resize((800.0, 600.0));
    render.update(&root, &mut rsc);
    rsc.ui.widgets.get_mut(&scroll).unwrap().scroll(0.0);
    render.update(&root, &mut rsc);

    let target = &rects[2];
    let before = render.resolved_region(target, &rsc).unwrap();

    rsc.ui.widgets.get_mut(&scroll).unwrap().scroll(-37.0);
    render.update(&root, &mut rsc);

    let after = render.resolved_region(target, &rsc).unwrap();
    let before_px = before.to_px((800.0, 600.0).into());
    let after_px = after.to_px((800.0, 600.0).into());

    // Scrolling by -37 moves `amt` from 0 to 37, sliding the content's
    // top-left up by 37px -- `resolved_region` (the CPU twin of the vertex
    // shader's chain walk) must reflect that immediately, not the
    // pre-scroll position, or a tap routed through it would land on
    // whatever is now at the old coordinates instead of this widget.
    assert!(
        (after_px.top_left.y - (before_px.top_left.y - 37.0)).abs() < 0.01,
        "before={before_px:?} after={after_px:?}"
    );
}

/// `ActiveData::mask` is the mask a widget was drawn **under**, not the one
/// it set for itself -- `redraw` feeds it straight back in as the inherited
/// mask, so storing the set one hands a `Masked` its own mask the second
/// time round -- which `Painter::set_mask` asserts against, since a mask
/// that chains to itself is a clip loop. That was an abort the first time
/// the composer's new scroll area was redrawn on the emulator; a targeted
/// redraw of a `Masked` is what any real screen does whenever anything
/// inside it changes.
#[test]
fn redrawing_a_masked_widget_does_not_nest_its_own_mask() {
    let mut rsc = TestRsc {
        ui: UiData::default(),
    };
    let (_scroll, inner_root, _rects) = scrolled_rects(&mut rsc, 8);
    let masked = rsc.ui.widgets.add_strong(Masked {
        shape: None,
        inner: inner_root,
    });
    let masked_id = masked.id();
    let root = masked.any();
    let mut render = UiRenderState::new();
    render.resize((800.0, 600.0));
    render.update(&root, &mut rsc);

    render.redraw(masked_id, &mut rsc);
    render.redraw(masked_id, &mut rsc);

    assert_eq!(
        render.active.get(&masked_id).unwrap().mask,
        MaskIdx::NONE,
        "a `Masked` at the root is drawn under no mask of its own"
    );
}

#[test]
fn a_mask_stays_put_while_its_scrolled_content_moves() {
    let mut rsc = TestRsc {
        ui: UiData::default(),
    };
    let (scroll, inner_root, _rects) = scrolled_rects(&mut rsc, 500);
    let masked = rsc.ui.widgets.add_strong(Masked {
        shape: None,
        inner: inner_root,
    });
    let masked_id = masked.id();
    let root = masked.any();
    let mut render = UiRenderState::new();
    render.resize((800.0, 600.0));

    render.update(&root, &mut rsc);
    rsc.ui.widgets.get_mut(&scroll).unwrap().scroll(0.0);
    render.update(&root, &mut rsc);

    let masked_slot_before = render.active.get(&masked_id).unwrap().move_slot;
    let mask_delta_before = rsc.ui.move_offsets[masked_slot_before.idx()].delta;

    rsc.ui.widgets.get_mut(&scroll).unwrap().scroll(-40.0);
    render.update(&root, &mut rsc);

    let masked_slot_after = render.active.get(&masked_id).unwrap().move_slot;
    let mask_delta_after = rsc.ui.move_offsets[masked_slot_after.idx()].delta;

    // `Masked` itself is never the target of a `mov`/`reposition` here --
    // only its scrolled child is -- so the slot its own mask references
    // (`Painter::set_mask` bakes in `self.move_slot`, i.e. this one) must
    // still read zero after the scroll. The visible counterpart of this
    // (the clipped edge follows the scroll while the viewport border does
    // not) is `iris/run-headless.sh`'s job to catch in a real frame; this
    // is the numeric half, on the same data the fragment shader's
    // `resolve_move` reads. See LAYOUT.md section 2b.
    assert_eq!(mask_delta_before, [0.0, 0.0]);
    assert_eq!(mask_delta_after, [0.0, 0.0]);
}

/// Reproduces `transcript_ui::composer::build_composer`'s exact tree shape
/// (a `Rect` background stacked behind a `Span::RIGHT`-wrapped, padded,
/// `rest`-width `TextEdit`, itself the second child of an outer
/// `Span::DOWN` beside a `rest(1)`-height sibling) without the event/
/// resource plumbing `composer.rs`'s builders need, to isolate whether the
/// bug Iris reported on 2026-09-06 ("text seems to not appear in box")
/// is this crate's layout engine or something specific to the real
/// composer/screen. `TextEditable::edit` only needs `UiRsc`, so a plain
/// insert exercises the exact redraw path a keystroke does.
fn composer_like_tree(rsc: &mut TestRsc) -> (WeakWidget<TextEdit>, StrongWidget) {
    let field = wtext("")
        .editable(EditMode::MultiLine)
        .text_align(Align::LEFT)
        .wrap(true)
        .size(18)
        .color(UiColor::WHITE)
        .add(rsc);
    let bar = (field.pad(dp(12)).width(rest(1)),)
        .span(Dir::RIGHT)
        .background(rect(UiColor::new(40, 40, 46, 255)))
        .add(rsc);
    let list_stand_in = rect(UiColor::BLACK).height(rest(1)).add(rsc);
    let tree = (list_stand_in, bar).span(Dir::DOWN).add_strong(rsc).any();
    (field, tree)
}

/// The reproduction itself. A window this tall stands in for the keyboard
/// closed; the second, shorter `resize` stands in for `adjustResize`
/// shrinking the surface when the IME opens -- exactly the sequence
/// `IrisViewPeer::surface_changed` drives on a real keyboard open. Typing
/// happens both before and after, since Iris's report was specifically
/// that text typed *after* the keyboard was already up did not appear.
#[test]
fn composing_text_after_a_keyboard_resize_lands_in_the_bars_own_region() {
    let mut rsc = TestRsc {
        ui: UiData::default(),
    };
    let (field, root) = composer_like_tree(&mut rsc);
    let mut render = UiRenderState::new();

    render.resize((1080.0, 2298.0));
    render.update(&root, &mut rsc);
    // Focusing a field is what places its caret on a real tap
    // (`attr.rs`'s `on_press` -> `TextEditCtx::select`), and an insert
    // with no caret is a routing bug rather than a state to simulate --
    // `insert_str`'s own `debug_assert!` says so, and caught this test
    // typing into an unfocused field when it was added.
    field
        .edit(&mut rsc)
        .select(vec2(40.0, 2250.0), vec2(1080.0, 2298.0), false, false);
    field.edit(&mut rsc).insert("a");
    render.update(&root, &mut rsc);

    let before_px = render.window_region(&field, &rsc).unwrap();
    // The field is one line plus 12dp of padding on a 2298-tall window --
    // nowhere near the whole window's height, and anchored at the bottom.
    assert!(
        before_px.bot_right.y - before_px.top_left.y < 200.0,
        "before a resize: {before_px:?}"
    );
    assert!(
        before_px.top_left.y > 1800.0,
        "expected the bar near the bottom before a resize: {before_px:?}"
    );

    // The keyboard opens: a real `surface_changed`/`resize` to a shorter
    // window, then a further keystroke -- the redraw that must land in the
    // bar's new (also short) region, not whatever region a provisional
    // measurement pass used along the way.
    render.resize((1080.0, 1478.0));
    render.update(&root, &mut rsc);
    field.edit(&mut rsc).insert("b");
    render.update(&root, &mut rsc);

    let after_px = render.window_region(&field, &rsc).unwrap();
    assert!(
        after_px.bot_right.y - after_px.top_left.y < 200.0,
        "after a resize + keystroke: {after_px:?}"
    );
    assert!(
        after_px.top_left.y > 1200.0,
        "expected the bar near the bottom of the shorter window: {after_px:?}"
    );
}

/// `ScrollArea` used to be documented as resolving its own lengths against
/// `Painter::output_size` -- the window -- which read as if a scroll area
/// smaller than the screen could not work, and cost a session's
/// investigation before the composer was wired up (docs/RUST.md,
/// 2026-09-06). It measures `painter.px_size()` now, so this pins the
/// three numbers that follow from the offered box: what it reports
/// upward, what its capping parent reports, and how far it can pan.
#[test]
fn a_scroll_measures_the_box_it_was_offered_not_the_window() {
    let mut rsc = TestRsc {
        ui: UiData::default(),
    };
    let rect = rsc.ui.widgets.add_strong(Rect::new(UiColor::WHITE));
    let tall = rsc.ui.widgets.add_strong(Sized {
        inner: rect.any(),
        x: None,
        y: Some(Len::abs(1000.0)),
    });
    let scroll = rsc
        .ui
        .widgets
        // Start-anchored, so the `scroll(-37.0)` below has somewhere to
        // go -- see `scrolled_rects`' note on the same choice.
        .add_strong(ScrollArea::new(tall.any(), Axis::Y, Pin::Start));
    let scroll_w = scroll.weak();
    let scroll_id = scroll.id();
    let capped = rsc.ui.widgets.add_strong(MaxSize {
        inner: scroll.any(),
        x: None,
        y: Some(Len::abs(100.0)),
    });
    let capped_id = capped.id();
    let root = capped.any();

    let mut render = UiRenderState::new();
    render.resize((800.0, 600.0));
    // Two passes: the first offers the content a zero-length region
    // (nothing measured yet) and learns the real content length from what
    // comes back -- see `scrolling_moves_in_o1_without_a_redraw` for why
    // that warm-up is deliberate rather than a bug.
    render.update(&root, &mut rsc);
    rsc.ui.widgets.get_mut(&scroll_w).unwrap().scroll(0.0);
    render.update(&root, &mut rsc);

    // Reports the *content*, so the cap above it has something to cap;
    // reporting the container instead would make the answer a function of
    // itself, since the container is sized from this very number.
    assert_eq!(
        render.active.get(&scroll_id).unwrap().size.y,
        Len::abs(1000.0)
    );
    assert_eq!(
        render.active.get(&capped_id).unwrap().size.y,
        Len::abs(100.0),
        "the cap, not the content and not the window"
    );

    // Panning is bounded by content minus *container*: 900, not the 400
    // a 600px window would give. The draw is what spends the delta -- a
    // controller banks it until the layout that knows where the content
    // ends (`ScrollController::take_delta`).
    rsc.ui.widgets.get_mut(&scroll_w).unwrap().scroll(-10_000.0);
    render.update(&root, &mut rsc);
    assert!(
        (rsc.ui.widgets.get_mut(&scroll_w).unwrap().amt() - 900.0).abs() < 0.01,
        "amt={}",
        rsc.ui.widgets.get_mut(&scroll_w).unwrap().amt()
    );
}

/// The half `hit_testing_follows_a_scrolled_widget` could not see: it
/// checks a *descendant* of the widget `ScrollArea` actually moves, whose own
/// `region` is stale and is corrected entirely by the move chain. The
/// moved widget itself had its `region` updated *and* the chain delta
/// added on top, so its hit box sat at twice the pan -- which is why a
/// finger pan of the composer left its field untappable. See
/// `ActiveData::move_applied`.
#[test]
fn a_panned_widgets_own_hit_box_moves_exactly_once() {
    let mut rsc = TestRsc {
        ui: UiData::default(),
    };
    let rect = rsc.ui.widgets.add_strong(Rect::new(UiColor::WHITE));
    let tall = rsc.ui.widgets.add_strong(Sized {
        inner: rect.any(),
        x: None,
        y: Some(Len::abs(1000.0)),
    });
    let tall_w = tall.weak();
    let scroll = rsc
        .ui
        .widgets
        // Start-anchored, so the `scroll(-37.0)` below has somewhere to
        // go -- see `scrolled_rects`' note on the same choice.
        .add_strong(ScrollArea::new(tall.any(), Axis::Y, Pin::Start));
    let scroll_w = scroll.weak();
    let root = scroll.any();

    let mut render = UiRenderState::new();
    render.resize((800.0, 600.0));
    render.update(&root, &mut rsc);
    rsc.ui.widgets.get_mut(&scroll_w).unwrap().scroll(0.0);
    render.update(&root, &mut rsc);

    let before = render.window_region(&tall_w, &rsc).unwrap();
    rsc.ui.widgets.get_mut(&scroll_w).unwrap().scroll(-37.0);
    render.update(&root, &mut rsc);
    let after = render.window_region(&tall_w, &rsc).unwrap();

    assert!(
        (after.top_left.y - (before.top_left.y - 37.0)).abs() < 0.01,
        "the pan was applied twice: before={before:?} after={after:?}"
    );
}

/// A `Masked` used to allocate a **new** mask slot on every draw, and
/// `draw_inner`'s unchanged-region fast path means its descendants are
/// mostly *not* redrawn with it -- so they went on referencing the slot
/// they were first drawn under, whose region had since stopped being the
/// widget's. Measured 2026-09-06 on the composer's tree: four live mask
/// entries, none of them the `Masked`'s current box, and the field it was
/// meant to clip drew nothing at all on the emulator. The slot is
/// allocated once and rewritten in place now (`ActiveData::own_mask`), so
/// this pins both halves: one entry, and that entry is the widget's own
/// region.
#[test]
fn a_masked_widget_keeps_one_mask_slot_that_is_always_its_own_region() {
    let mut rsc = TestRsc {
        ui: UiData::default(),
    };
    let (_scroll, inner_root, _rects) = scrolled_rects(&mut rsc, 8);
    let masked = rsc.ui.widgets.add_strong(Masked {
        shape: None,
        inner: inner_root,
    });
    let masked_id = masked.id();
    // Placed at the bottom of a `Span::DOWN` behind a `rest(1)` sibling,
    // which is what moves the bar away from the provisional slot it is
    // first drawn at -- the move that left the stale mask behind.
    let filler = rsc.ui.widgets.add_strong(Rect::new(UiColor::BLACK));
    let filler = rsc.ui.widgets.add_strong(Sized {
        inner: filler.any(),
        x: None,
        y: Some(rest(1)),
    });
    let capped = rsc.ui.widgets.add_strong(MaxSize {
        inner: masked.any(),
        x: None,
        y: Some(Len::abs(60.0)),
    });
    let mut span = Span::empty(Dir::DOWN);
    span.push(filler.any());
    span.push(capped.any());
    let root = rsc.ui.widgets.add_strong(span).any();

    let mut render = UiRenderState::new();
    render.resize((800.0, 600.0));
    for _ in 0..3 {
        render.update(&root, &mut rsc);
        render.redraw(masked_id, &mut rsc);
    }

    assert_eq!(
        rsc.ui.masks.iter().count(),
        1,
        "one `Masked` must own exactly one mask slot, however often it is redrawn"
    );
    let mask = *rsc.ui.masks.iter().next().unwrap();
    assert_eq!(
        render.primitives.instance(mask.primitive).region,
        render.active.get(&masked_id).unwrap().region,
        "the mask a descendant clips against must be this widget's current box"
    );
}

/// A `dp` cap that has done its job must be reported in pixels. `Span`
/// places a child using the `abs`/`rel` of the length it reported, so a
/// `MaxSize` handing back the caller's own `dp(168)` gave the composer's
/// bar a slot of **zero** the moment its content grew past six lines --
/// and the `ScrollArea` inside then measured its container at -63px (the
/// padding, subtracted from nothing) and panned the whole message out of
/// view. Measured on this checkout's emulator, 2026-09-06:
/// `container=-63 content=415.8 amt=478.8`. See `Len::fold_dp`.
#[test]
fn a_dp_cap_is_reported_in_pixels_so_a_span_can_place_it() {
    let mut rsc = TestRsc {
        ui: UiData::default(),
    };
    let rect = rsc.ui.widgets.add_strong(Rect::new(UiColor::WHITE));
    let tall = rsc.ui.widgets.add_strong(Sized {
        inner: rect.any(),
        x: None,
        y: Some(Len::abs(1000.0)),
    });
    let capped = rsc.ui.widgets.add_strong(MaxSize {
        inner: tall.any(),
        x: None,
        y: Some(Len::dp(100.0)),
    });
    let capped_w = capped.weak();
    let filler = rsc.ui.widgets.add_strong(Rect::new(UiColor::BLACK));
    let filler = rsc.ui.widgets.add_strong(Sized {
        inner: filler.any(),
        x: None,
        y: Some(rest(1)),
    });
    let mut span = Span::empty(Dir::DOWN);
    span.push(filler.any());
    span.push(capped.any());
    let root = rsc.ui.widgets.add_strong(span).any();

    let mut render = UiRenderState::new();
    render.resize((800.0, 600.0));
    render.set_density(2.5);
    render.update(&root, &mut rsc);
    render.update(&root, &mut rsc);

    let box_px = render.window_region(&capped_w, &rsc).unwrap();
    let height = box_px.bot_right.y - box_px.top_left.y;
    assert!(
        (height - 250.0).abs() < 0.01,
        "expected the 100dp cap at density 2.5 to be a 250px slot, got {height} ({box_px:?})"
    );
}

/// The sibling of `a_panned_widgets_own_hit_box_moves_exactly_once`, on
/// the branch that fix had no reason to touch: `draw_inner`'s
/// size-independent fast path rewrites a widget's primitives *in place*
/// and leaves its move slot alone, so unlike `mov` there is no slot delta
/// for `region` to have absorbed. Counting one there anyway makes
/// `resolved_region` subtract a delta the chain never held, and the
/// widget's hit box lands short of where it is drawn by exactly the
/// distance it just moved -- with nothing on screen to say so, since the
/// primitives are in the right place.
#[test]
fn a_size_independent_widget_moved_by_its_parent_has_the_hit_box_it_is_drawn_at() {
    let mut rsc = TestRsc {
        ui: UiData::default(),
    };
    let top = rsc.ui.widgets.add_strong(Rect::new(UiColor::WHITE));
    let spacer = rsc.ui.widgets.add_strong(Sized {
        inner: top.any(),
        x: None,
        y: Some(Len::abs(100.0)),
    });
    let spacer_w = spacer.weak();
    // `Rect` is `is_size_independent`, so growing the spacer above it
    // offers this one a region that changed *both* position and size --
    // the one shape that reaches the branch under test.
    let below = rsc.ui.widgets.add_strong(Rect::new(UiColor::WHITE));
    let below_w = below.weak();
    let mut span = Span::empty(Dir::DOWN);
    span.push(spacer.any());
    span.push(below.any());
    let root = rsc.ui.widgets.add_strong(span).any();

    let mut render = UiRenderState::new();
    render.resize((800.0, 600.0));
    render.update(&root, &mut rsc);
    // `Span` draws each child once at the full region to measure it and
    // then places it, so this widget has already been through the branch
    // once by the end of the very first frame.
    let first = render.window_region(&below_w, &rsc).unwrap();
    assert!(
        (first.top_left.y - 100.0).abs() < 0.01,
        "hit box at {:?}, drawn at y=100",
        first.top_left
    );

    rsc.ui.widgets.get_mut(&spacer_w).unwrap().y = Some(Len::abs(250.0));
    render.update(&root, &mut rsc);
    let after = render.window_region(&below_w, &rsc).unwrap();
    assert!(
        (after.top_left.y - 250.0).abs() < 0.01,
        "hit box at {:?}, drawn at y=250",
        after.top_left
    );
}

/// A parent that both `mov`s a child (its own layout moved the box it
/// offers) and `reposition`s it inside that box in the same frame -- what
/// `LazySpan::place`'s Bottom-known branch does once a row's cached height
/// stops matching what the row reports, which is reachable as soon as a
/// transcript row's blocks wrap (docs/IRIS_TODO.md's "Found by P1a").
struct MoveThenPlace {
    inner: StrongWidget,
    /// Where the child is *offered* a (constant-size) box, moved between
    /// frames by the test.
    offer_top: f32,
    /// Where the child is then placed within this widget's own region.
    place_top: f32,
}

impl Widget for MoveThenPlace {
    fn draw(&mut self, painter: &mut Painter) -> Size {
        let offer = UiRegion::new(
            UiSpan::FULL,
            UiSpan::new(
                UiScalar::abs(self.offer_top),
                UiScalar::abs(self.offer_top + 40.0),
            ),
        );
        painter.widget_within(&self.inner, offer);
        let place = UiRegion::new(
            UiSpan::FULL,
            UiSpan::new(
                UiScalar::abs(self.place_top),
                UiScalar::abs(self.place_top + 40.0),
            ),
        );
        painter.reposition(&self.inner, place);
        Size::default()
    }
}

/// `mov` accumulates a delta onto a widget's move slot and `reposition`
/// overwrites it, and both can legitimately land on one widget in one
/// frame (see `MoveThenPlace`). `reposition` used to write its own delta
/// alone, which dropped the move and put the child back at the position
/// the offered box had *before* it moved; a `debug_assert!` that
/// `move_applied` was zero hid that behind a panic instead of fixing it.
/// The slot has one owner and one meaning now --
/// `move_applied + repositioned` -- so the child stays where it was
/// placed however its offered box moves. Fails at the offer's position
/// (200) rather than the placement's (100) without that.
#[test]
fn a_widget_moved_by_its_parent_and_then_placed_inside_it_lands_at_the_placement() {
    let mut rsc = TestRsc {
        ui: UiData::default(),
    };
    let rect = rsc.ui.widgets.add_strong(Rect::new(UiColor::WHITE));
    let child = rsc.ui.widgets.add_strong(Sized {
        inner: rect.any(),
        x: None,
        y: Some(Len::abs(40.0)),
    });
    let child_w = child.weak();
    let parent = rsc.ui.widgets.add_strong(MoveThenPlace {
        inner: child.any(),
        offer_top: 0.0,
        place_top: 100.0,
    });
    let parent_w = parent.weak();
    let root = parent.any();

    let mut render = UiRenderState::new();
    render.resize((200.0, 400.0));
    render.update(&root, &mut rsc);
    let before = render.window_region(&child_w, &rsc).unwrap();
    assert!(
        (before.top_left.y - 100.0).abs() < 0.01,
        "the child should be drawn where it was placed, not where it was offered: {before:?}"
    );

    // Move the offered box without changing its size (the `mov` fast path)
    // and place the child at the same spot as before. Marking the parent
    // dirty is what a real container's own content change does; the child
    // itself is untouched, which is the case `mov` exists for.
    {
        let parent = rsc.ui.widgets.get_mut(&parent_w).unwrap();
        parent.offer_top = 200.0;
    }
    rsc.ui.widgets.needs_redraw.insert(parent_w.id());
    render.update(&root, &mut rsc);
    let after = render.window_region(&child_w, &rsc).unwrap();
    assert!(
        (after.top_left.y - 100.0).abs() < 0.01,
        "the placement did not change, so neither should the child: before={before:?} \
         after={after:?}"
    );
}

// ---------------------------------------------------------------------
// LAYOUT.md's "Masks with a shape" -- its pass conditions, at layer 1.
//
// The shape a mask clips to is a *primitive already drawn*, never a copy
// of one, so "the child's clipped corner" and "the container's own corner"
// are the same arithmetic. These say so by evaluating both and demanding
// exact equality: an approximate assertion would also pass a second copy
// of the radius that merely happened to agree.
// ---------------------------------------------------------------------

const RADIUS: f32 = 20.0;

/// A rounded container with `.masked_by` it, holding a `Rect::REST` child
/// that fills it -- so the child's own corners are exactly the corners
/// being clipped away. Returns the drawn state, the mask, the child, and
/// the shape primitive the mask points at.
fn rounded_container(rsc: &mut TestRsc) -> (UiRenderState, MaskIdx, WidgetId, u32) {
    let child = rsc.ui.widgets.add_strong(Rect::new(UiColor::WHITE));
    let child_id = child.id();
    let shape = rsc
        .ui
        .widgets
        .add_strong(Rect::new(UiColor::BLACK).radius(Len::abs(RADIUS)));
    let shape_id = shape.id();
    let root = rsc
        .ui
        .widgets
        .add_strong(Masked {
            shape: Some(shape.any()),
            inner: child.any(),
        })
        .any();

    let mut render = UiRenderState::new();
    render.resize((200.0, 100.0));
    render.update(&root, rsc);

    let mask = render
        .active
        .get(&child_id)
        .expect("the child is drawn")
        .mask;
    assert_ne!(
        mask,
        MaskIdx::NONE,
        "the child was drawn with no clip at all"
    );
    let slot = render
        .first_primitive(shape_id)
        .expect("the shape widget drew a rect");
    (render, mask, child_id, slot)
}

/// The pass condition: the child's coverage at a corner pixel *equals*
/// the container's own coverage there. Exactly equal, because it is the
/// same primitive evaluated once -- LAYOUT.md's point 1.
#[test]
fn a_masked_child_is_clipped_by_its_container_s_own_corner() {
    let mut rsc = TestRsc {
        ui: UiData::default(),
    };
    let (render, mask, _child, slot) = rounded_container(&mut rsc);
    let corners = render.primitive_corners(slot, &rsc);
    let radius = render
        .primitives
        .primitive_data::<RectPrimitive>(slot)
        .expect("a mask's shape is a rect")
        .radius;

    // Across the whole corner arc, not one point on it: a single sample
    // is satisfied by a mask that clips to the box and happens to agree
    // where the two coincide. Swept from the arc's own centre -- the
    // straight chord between the two ends of the arc lies *inside* the
    // circle everywhere, so a walk along it never leaves the shape and
    // the `outside` count below is what caught that.
    let arc_center = corners.top_left + Vec2::new(radius, radius);
    let (mut outside, mut inside) = (0, 0);
    for i in 0..=20 {
        let angle = std::f32::consts::FRAC_PI_2 * i as f32 / 20.0;
        let dir = Vec2::new(-angle.cos(), -angle.sin());
        for out in [-1.5f32, 0.0, 1.5] {
            let pos = arc_center + dir * (radius + out);
            let container = rounded_rect_coverage(pos, corners.top_left, corners.bot_right, radius);
            assert_eq!(
                render.mask_coverage(mask, pos, &rsc),
                container,
                "at {pos:?} the child's clip and the container's own edge disagree",
            );
            if container < 0.5 {
                outside += 1;
            } else {
                inside += 1;
            }
        }
    }
    assert!(
        outside > 0 && inside > 0,
        "the sweep stayed on one side of the curve ({outside} out, {inside} in), so it proved \
         nothing about the corner"
    );
}

/// A hit test asks the same question the pixels do: the corner the
/// container rounded away is not there to be pressed, and a point just
/// inside the curve is. LAYOUT.md's point 4.
#[test]
fn a_mask_s_shape_decides_what_can_be_pressed() {
    let mut rsc = TestRsc {
        ui: UiData::default(),
    };
    let (render, mask, _child, slot) = rounded_container(&mut rsc);
    let corners = render.primitive_corners(slot, &rsc);

    // The very corner of the box, which the radius cut off.
    let cut = corners.top_left + Vec2::new(1.0, 1.0);
    assert!(
        !render.mask_admits(mask, cut, &rsc),
        "the corner the container rounded away is still pressable",
    );
    // The same distance in along the diagonal, past the curve.
    let inside = corners.top_left + Vec2::new(RADIUS, RADIUS);
    assert!(
        render.mask_admits(mask, inside, &rsc),
        "a point well inside the curve is not pressable",
    );
    // And the middle of an edge, which no radius touches -- the half the
    // rounding had no reason to change.
    let edge = Vec2::new(
        (corners.top_left.x + corners.bot_right.x) / 2.0,
        corners.top_left.y + 1.0,
    );
    assert!(
        render.mask_admits(mask, edge, &rsc),
        "a straight edge between two corners is not pressable",
    );
}

/// Nested masks multiply, so a pixel inside two feathered corners is
/// dimmed by both -- LAYOUT.md's point 2, and the "alpha should be
/// decreased / multiplied" Iris asked for. Written as a product of the
/// two the shader would compute separately, which is what "multiply"
/// means and what an intersection test would get wrong.
#[test]
fn nested_masks_multiply_their_coverage() {
    let mut rsc = TestRsc {
        ui: UiData::default(),
    };
    let child = rsc.ui.widgets.add_strong(Rect::new(UiColor::WHITE));
    let child_id = child.id();

    let inner_shape = rsc
        .ui
        .widgets
        .add_strong(Rect::new(UiColor::BLACK).radius(Len::abs(RADIUS)));
    let inner_shape_id = inner_shape.id();
    let inner = rsc.ui.widgets.add_strong(Masked {
        shape: Some(inner_shape.any()),
        inner: child.any(),
    });
    let outer_shape = rsc
        .ui
        .widgets
        .add_strong(Rect::new(UiColor::BLACK).radius(Len::abs(RADIUS)));
    let outer_shape_id = outer_shape.id();
    let root = rsc
        .ui
        .widgets
        .add_strong(Masked {
            shape: Some(outer_shape.any()),
            inner: inner.any(),
        })
        .any();

    let mut render = UiRenderState::new();
    render.resize((200.0, 100.0));
    render.update(&root, &mut rsc);

    let mask = render.active.get(&child_id).expect("drawn").mask;
    let one = |render: &UiRenderState, rsc: &TestRsc, id, pos| {
        let slot = render.first_primitive(id).expect("a shape rect");
        let c = render.primitive_corners(slot, rsc);
        let radius = render
            .primitives
            .primitive_data::<RectPrimitive>(slot)
            .unwrap()
            .radius;
        rounded_rect_coverage(pos, c.top_left, c.bot_right, radius)
    };

    // A point on the corner arc, where both feathers are partial -- the
    // only place a product and a minimum differ.
    let slot = render.first_primitive(inner_shape_id).unwrap();
    let corners = render.primitive_corners(slot, &rsc);
    let pos = corners.top_left + Vec2::new(RADIUS * 0.3, RADIUS * 0.3);
    let inner_cov = one(&render, &rsc, inner_shape_id, pos);
    let outer_cov = one(&render, &rsc, outer_shape_id, pos);
    assert!(
        inner_cov > 0.0 && inner_cov < 1.0,
        "the sample point is not inside a feather ({inner_cov}), so this proves nothing"
    );
    assert_eq!(
        render.mask_coverage(mask, pos, &rsc),
        inner_cov * outer_cov,
        "two nested masks must multiply, not intersect",
    );
}

/// A plain `.masked()` -- no shape given -- still clips to the widget's
/// own box with square corners, which is what every list and scroll area
/// relies on. The half the shape work had no reason to touch, and the one
/// that would silently round every existing clip if `set_mask` ever wrote
/// a radius of its own.
#[test]
fn a_plain_mask_still_clips_to_a_square_box() {
    let mut rsc = TestRsc {
        ui: UiData::default(),
    };
    let (_scroll, inner_root, _rects) = scrolled_rects(&mut rsc, 8);
    let root = rsc
        .ui
        .widgets
        .add_strong(Masked {
            shape: None,
            inner: inner_root,
        })
        .any();
    let mut render = UiRenderState::new();
    render.resize((200.0, 100.0));
    render.update(&root, &mut rsc);

    let mask = *rsc.ui.masks.iter().next().expect("one mask");
    let corners = render.primitive_corners(mask.primitive, &rsc);
    let mask_idx = MaskIdx::preset(0);
    assert!(
        render.mask_admits(mask_idx, corners.top_left + Vec2::new(0.5, 0.5), &rsc),
        "a square clip must admit its own corner pixel",
    );
    assert!(
        !render.mask_admits(mask_idx, corners.top_left - Vec2::new(2.0, 2.0), &rsc),
        "a square clip must reject a point outside it",
    );
}

/// A scroll area created to be *read* opens at the beginning of its
/// content, however many frames it takes to learn how long that content
/// is.
///
/// The bug this pins: `content_len` was `0.0` both for "nothing here" and
/// for "not drawn yet", so the first frame's clamp found a range of zero,
/// read `amt == len` as "sitting at the end", and set `snap_end` -- and
/// the frame after, now knowing the real length, jumped to it. On screen
/// that was a code fence opening at the end of its longest line, in the
/// middle of a word (`iris/run-headless.sh phone`, 2026-09-08).
#[test]
fn a_scroll_area_opens_at_the_start_of_content_it_has_not_measured_yet() {
    for (name, pin, want) in [("read", Pin::Start, 0.0), ("written", Pin::End, 4900.0)] {
        let mut rsc = TestRsc {
            ui: UiData::default(),
        };
        let fill = rsc.ui.widgets.add_strong(Rect::new(UiColor::WHITE)).any();
        let tall = rsc.ui.widgets.add_strong(Sized {
            inner: fill,
            x: None,
            y: Some(Len::abs(5000.0)),
        });
        let scroll = rsc
            .ui
            .widgets
            .add_strong(ScrollArea::new(tall.any(), Axis::Y, pin));
        let weak = scroll.weak();
        let root = scroll.any();

        let mut render = UiRenderState::new();
        render.resize((800.0, 100.0));
        // Twice: the first draw is the one that measures the content, and
        // the defect only showed on the second. The touch in between is
        // what asks for that second draw -- an unchanged frame draws
        // nothing at all, which is the point of the frame before it.
        render.update(&root, &mut rsc);
        let _ = rsc.ui.widgets.get_mut(&weak);
        render.update(&root, &mut rsc);

        let amt = rsc.ui.widgets.get(&weak).unwrap().amt();
        assert!(
            (amt - want).abs() < 0.01,
            "an area to be {name} should have opened at {want}, got {amt}"
        );
    }
}

/// docs/IRIS_TODO.md's "A `Span` of `Pad`ded children inside another
/// `Span` places those children a slot out of step", worked around in
/// `transcript-ui/src/tool.rs` by flattening the two spans into one --
/// which costs a tool group the inset its cards should sit inside.
///
/// The shape is the smallest one that reproduced it there: an outer
/// `Span(DOWN)` whose second child is another `Span(DOWN)` whose children
/// are each a `Pad` around a fixed-height rect. Each rect is asserted to
/// be *drawn* where its own box is -- `primitive_corners` rather than
/// `window_region`, since the report is about what is on screen and the
/// two resolve the move chain differently.
#[test]
fn a_span_of_padded_children_inside_a_span_draws_each_where_its_box_is() {
    const PAD: f32 = 4.0;
    const ROW: f32 = 20.0;
    const HEADER: f32 = 30.0;
    let mut rsc = TestRsc {
        ui: UiData::default(),
    };
    let header_fill = rsc.ui.widgets.add_strong(Rect::new(UiColor::RED)).any();
    let header_id = header_fill.id();
    let header = rsc.ui.widgets.add_strong(Sized {
        inner: header_fill,
        x: None,
        y: Some(Len::abs(HEADER)),
    });
    let mut inner = Span::empty(Dir::DOWN);
    let mut rects = Vec::new();
    for _ in 0..3 {
        let rect = rsc.ui.widgets.add_strong(Rect::new(UiColor::WHITE));
        rects.push(rect.weak());
        let sized = rsc.ui.widgets.add_strong(Sized {
            inner: rect.any(),
            x: None,
            y: Some(Len::abs(ROW)),
        });
        let padded = rsc.ui.widgets.add_strong(Pad {
            padding: Padding::uniform(PAD),
            inner: sized.any(),
        });
        let fill = rsc.ui.widgets.add_strong(Rect::new(UiColor::BLUE)).any();
        let card = rsc.ui.widgets.add_strong(Stack {
            children: vec![fill, padded.any()],
            size: StackSize::Child(1),
        });
        let wide = rsc.ui.widgets.add_strong(Sized {
            inner: card.any(),
            x: Some(Len::rest(1.0)),
            y: None,
        });
        inner.push(wide.any());
    }
    let inner = rsc.ui.widgets.add_strong(inner);
    let outer = rsc.ui.widgets.add_strong(Span {
        children: vec![header.any(), inner.any()],
        dir: Dir::DOWN,
        gap: Len::ZERO,
    });
    let mut list = LazySpan::new(Dir::DOWN, Pin::End);
    list.push_back(LazyItem::new(0, outer.any()));
    let list = rsc.ui.widgets.add_strong(list);
    let root = rsc
        .ui
        .widgets
        .add_strong(Masked {
            shape: None,
            inner: list.any(),
        })
        .any();

    let mut render = UiRenderState::new();
    render.resize((200.0, 400.0));
    render.update(&root, &mut rsc);
    render.update(&root, &mut rsc);

    let head_slot = render
        .first_primitive(header_id)
        .expect("the header drew a primitive");
    let head_top = render.primitive_corners(head_slot, &rsc).top_left.y;
    for (i, rect) in rects.iter().enumerate() {
        let want = head_top + HEADER + (ROW + 2.0 * PAD) * i as f32 + PAD;
        let slot = render
            .first_primitive(rect.id())
            .expect("each rect drew a primitive");
        let drawn = render.primitive_corners(slot, &rsc);
        assert!(
            (drawn.top_left.y - want).abs() < 0.01,
            "row {i} should be drawn at y={want}, got {drawn:?}"
        );
    }
}