iris/tests/cases/retained.rs

//! What a second frame draws again, and what it keeps.

use std::{cell::Cell, rc::Rc};

use iris::harness::{Harness, assert_corners};
use iris::prelude::*;

/// A leaf that counts its draws and reports whatever size it is given, so a
/// test can see what the retained path skipped. One that reads its box in
/// pixels has a drawing that holds for that box alone.
struct Counted {
    draws: Rc<Cell<usize>>,
    size: Size,
    reads_box: bool,
}

impl Widget for Counted {
    fn draw(&mut self, painter: &mut Painter) -> Size {
        self.draws.set(self.draws.get() + 1);
        if self.reads_box {
            painter.px_size();
        }
        self.size
    }
}

struct Counts(Rc<Cell<usize>>);

impl Counts {
    fn get(&self) -> usize {
        self.0.get()
    }
}

fn counted(h: &mut Harness, size: Size, reads_box: bool) -> (WeakWidget<Counted>, Counts) {
    let draws = Rc::new(Cell::new(0));
    let id = Counted {
        draws: draws.clone(),
        size,
        reads_box,
    }
    .add(&mut h.rsc);
    (id, Counts(draws))
}

struct Layered {
    children: [StrongWidget<Rect>; 2],
    _revision: usize,
}

impl Widget for Layered {
    fn draw(&mut self, painter: &mut Painter) -> Size {
        painter.child_layer();
        painter.widget(&self.children[0]);
        painter.next_layer();
        painter.widget(&self.children[1]);
        Size::default()
    }
}

#[test]
fn a_redrawn_layered_widget_keeps_the_layer_it_was_entered_on() {
    let mut h = Harness::new((400, 200));
    let children = [
        rect(Color::RED).add_strong(&mut h.rsc),
        rect(Color::BLUE).add_strong(&mut h.rsc),
    ];
    let root = Layered {
        children,
        _revision: 0,
    }
    .add(&mut h.rsc);
    h.set_root(root);

    h.rsc[root]._revision += 1;
    h.frame();

    let label = h.rsc.widgets().label(root.id());
    let active = h
        .render
        .debug(h.rsc.widgets(), label)
        .find(|active| active.id == root.id())
        .unwrap();
    assert_eq!(active.layer, 0);
}

/// A fixed-width leaf beside one that takes what is left over, so changing
/// the first hands the second a different box without the output changing.
fn pair(h: &mut Harness, reads_box: bool) -> (WeakWidget<Counted>, Counts, WidgetId) {
    let (first, _) = counted(h, Size::from((100, 200)), false);
    let (second, draws) = counted(h, Size::LEFTOVER, reads_box);
    h.set_root((first, second).span(Dir::RIGHT));
    (first, draws, second.id())
}

#[test]
fn a_leaf_that_ignores_its_box_is_not_drawn_again_when_the_box_changes() {
    let mut h = Harness::new((400, 200));
    let (first, draws, second) = pair(&mut h, false);
    let settled = draws.get();
    assert_corners!(h, second, (100, 0), (400, 200));

    h.rsc[first].size = Size::from((150, 200));
    h.frame();

    assert_eq!(
        draws.get(),
        settled,
        "its box is a field to write, not a reason to draw"
    );
    assert_corners!(h, second, (150, 0), (400, 200));
}

#[test]
fn moving_an_ordinary_subtree_remaps_its_mask() {
    let mut h = Harness::new((400, 200));
    let (first, _) = counted(&mut h, Size::from((100, 200)), false);
    let inner = rect(Color::BLUE).add(&mut h.rsc);
    let masked = inner.masked().add(&mut h.rsc);
    h.set_root((first, masked).span(Dir::RIGHT));

    h.rsc[first].size = Size::from((150, 200));
    h.frame();

    let active = &h.render.active[&masked.id()];
    assert_eq!(
        h.rsc.ui().masks[active.mask.idx()].region,
        UiRegion::new(UiSpan::new(Len::px(150.0), Len::rel_max()), UiSpan::FULL,)
    );
    assert_corners!(h, inner, (150, 0), (400, 200));
}

#[test]
fn a_leaf_that_depends_on_its_box_is_drawn_again_when_the_box_changes() {
    let mut h = Harness::new((400, 200));
    let (first, draws, second) = pair(&mut h, true);
    let settled = draws.get();

    h.rsc[first].size = Size::from((150, 200));
    h.frame();

    // The preceding fixed child makes the remaining box this child's real
    // box, so measuring it also draws it in its final box.
    assert_eq!(draws.get(), settled + 1);
    assert_corners!(h, second, (150, 0), (400, 200));
}

#[test]
fn a_span_child_that_declares_its_length_is_drawn_once() {
    let mut h = Harness::new((400, 200));
    let (told, told_draws) = counted(&mut h, Size::from((100, 200)), false);
    let (asked, asked_draws) = counted(&mut h, Size::from((100, 200)), true);
    // The span takes one child's length from its hint and has to draw the
    // other to find out, so only the second is drawn before its final box.
    let hinted = told.width(100).add(&mut h.rsc);
    h.set_root((hinted, asked).span(Dir::RIGHT));

    assert_eq!(told_draws.get(), 1);
    // Reading its box makes its drawing hold for the measuring box alone,
    // and it reports less than that box: so it is drawn again in the box its
    // answer places it in, and once more in the final box the span chooses.
    // A widget that says what it holds for, as text does, skips the middle
    // one.
    assert_eq!(
        asked_draws.get(),
        3,
        "drawn to be measured, in its placed box, then in its final box"
    );
}

#[test]
fn a_span_relays_out_when_a_child_it_measured_changes() {
    let mut h = Harness::new((400, 200));
    let (first, _, second) = pair(&mut h, false);

    h.rsc[first].size = Size::from((250, 200));
    h.frame();

    assert_corners!(h, first, (0, 0), (250, 200));
    assert_corners!(h, second, (250, 0), (400, 200));
}

#[test]
fn a_repaint_that_keeps_its_size_does_not_relay_out() {
    let mut h = Harness::new((400, 200));
    let (first, draws) = counted(&mut h, Size::from((100, 200)), false);
    let (second, _) = counted(&mut h, Size::LEFTOVER, false);
    h.set_root((first, second).span(Dir::RIGHT));
    let settled = draws.get();

    // Taking mutable access is the ordinary content-change signal. This
    // widget returns the same size, so the parent has nothing to lay out.
    let _ = h.rsc.widgets_mut().get_dyn_mut(first.id());
    h.frame();

    assert_eq!(draws.get(), settled + 1);
}

#[test]
fn a_span_child_survives_the_next_frame() {
    let mut h = Harness::new((400, 200));
    // Both children declare a length, so the span chooses their boxes from
    // hints rather than drawing them to find out.
    let top = rect(Color::RED).height(80).add(&mut h.rsc);
    let bottom = rect(Color::BLUE).height(120).add(&mut h.rsc);
    h.set_root((top, bottom).span(Dir::DOWN));

    h.rsc.widgets_mut().get_dyn_mut(top.id());
    h.frame();

    assert_corners!(h, top, (0, 0), (400, 80));
    assert_corners!(h, bottom, (0, 80), (400, 200));
}

/// Lays its child out from the hint alone, never reading what it drew.
struct FromHint {
    inner: StrongWidget,
}

impl Widget for FromHint {
    fn draw(&mut self, painter: &mut Painter) -> Size {
        let len = painter.size_hint(&self.inner, Axis::Y).unwrap();
        let mut region = UiRegion::FULL;
        region.y.end = region.y.start.offset(len.px);
        painter.widget_within(&self.inner, region);
        Size::LEFTOVER
    }
}

#[test]
fn a_parent_that_only_read_a_hint_relays_out_when_the_hint_changes() {
    let mut h = Harness::new((400, 200));
    let inner = rect(Color::RED).height(80).add(&mut h.rsc);
    let parent = FromHint {
        inner: inner.add_strong(&mut h.rsc),
    }
    .add(&mut h.rsc);
    h.set_root(parent);
    assert_corners!(h, inner, (0, 0), (400, 80));

    h.set_len(inner, Axis::Y, 120);
    h.frame();

    assert_corners!(h, inner, (0, 0), (400, 120));
}

/// Reads its box's size, which nothing but its own draw can put right.
struct ReadsBox {
    draws: Rc<Cell<usize>>,
}

impl Widget for ReadsBox {
    fn draw(&mut self, painter: &mut Painter) -> Size {
        self.draws.set(self.draws.get() + 1);
        Size::from_px(painter.px_size().div_int(4))
    }
}

/// Reads its box across one axis only, so its drawing holds for a taller
/// box on its own and only a wider one is worth a draw.
///
/// Both of these report a quarter of what they read, without saying that the
/// drawing holds there too, so each length they are asked at costs two draws:
/// one to answer, and one in the quarter-sized box that answer places them
/// in. The counts below are in those pairs.
struct ReadsWidth {
    draws: Rc<Cell<usize>>,
}

impl Widget for ReadsWidth {
    fn draw(&mut self, painter: &mut Painter) -> Size {
        self.draws.set(self.draws.get() + 1);
        Size::from_px(PxVec2::new(
            painter.px_len(Axis::X).div_int(4),
            Px::from_int(20),
        ))
    }
}

#[test]
fn a_resize_does_not_redraw_what_the_shader_can_move() {
    let mut h = Harness::new((400, 200));
    let (leaf, draws) = counted(&mut h, Size::LEFTOVER, false);
    h.set_root(leaf);
    let settled = draws.get();

    h.resize((800, 100));
    assert!(h.needs_redraw());
    h.frame();

    assert_eq!(
        draws.get(),
        settled,
        "a scaling drawing follows its box, and the output is one"
    );
    assert_corners!(h, leaf, (0, 0), (800, 100));
}

#[test]
fn a_span_ruled_across_itself_moves_its_child_without_redrawing_it() {
    let mut h = Harness::new((400, 200));
    let (leaf, draws) = counted(&mut h, Size::LEFTOVER, false);
    let span = (leaf,).span(Dir::RIGHT).height(rel(1.0)).add(&mut h.rsc);
    h.set_root(span);
    let settled = draws.get();

    h.resize((400, 100));
    h.frame();

    assert_eq!(draws.get(), settled);
    assert_corners!(h, leaf, (0, 0), (400, 100));
    assert_eq!(h.render.active[&span.id()].size.y, LayoutLen::rel(1.0));
}

/// The output is the root of the box chain, so a resize is a box that changed
/// length like any other -- there is not a second rule for the window. A
/// drawing that holds for one length is drawn again whichever box moved.
#[test]
fn a_resize_redraws_what_does_not_scale() {
    let mut h = Harness::new((400, 200));
    let (leaf, draws) = counted(&mut h, Size::LEFTOVER, true);
    h.set_root(leaf);
    let settled = draws.get();

    h.resize((800, 100));
    h.frame();

    assert_eq!(draws.get(), settled + 1, "its box is a different length");
    assert_corners!(h, leaf, (0, 0), (800, 100));
}

#[test]
fn a_resize_redraws_what_read_its_box() {
    let mut h = Harness::new((400, 200));
    let draws = Rc::new(Cell::new(0));
    let leaf = ReadsBox {
        draws: draws.clone(),
    }
    .add(&mut h.rsc);
    h.set_root(leaf);
    let settled = draws.get();

    h.resize((800, 100));
    h.frame();

    assert_eq!(draws.get(), settled + 2);
}

#[test]
fn a_resize_only_redraws_read_axes() {
    let mut h = Harness::new((400, 200));
    let draws = Rc::new(Cell::new(0));
    let leaf = ReadsWidth {
        draws: draws.clone(),
    }
    .add(&mut h.rsc);
    h.set_root(leaf);
    let settled = draws.get();

    h.resize((400, 300));
    h.frame();
    assert_eq!(draws.get(), settled, "height was never read");

    h.resize((800, 300));
    h.frame();
    assert_eq!(draws.get(), settled + 2, "width changes its answer");
}

/// A window is measured onto the grid like everything else, so a resize too
/// small to reach the next step is not a resize at all -- and one that does
/// reach it is, however little of a pixel it is worth.
#[test]
fn a_resize_within_one_step_is_not_a_resize() {
    let mut h = Harness::new((400, 200));
    let draws = Rc::new(Cell::new(0));
    let leaf = ReadsWidth {
        draws: draws.clone(),
    }
    .add(&mut h.rsc);
    h.set_root(leaf);
    let settled = draws.get();

    // All of these are 400 px to the nearest step.
    let step = Px::STEP.to_f32();
    for part in [0.1, 0.2, 0.3] {
        h.resize((400.0 + step * part, 200.0));
        h.frame();
        assert_eq!(draws.get(), settled);
    }

    h.resize((400.0 + step, 200.0));
    h.frame();
    assert_eq!(draws.get(), settled + 2);
}

/// The same for a box that changes because a sibling did: what is compared
/// is the length on the grid, and three lengths that land on one step are
/// one length.
#[test]
fn a_box_change_within_one_step_is_not_a_change() {
    let mut h = Harness::new((400, 200));
    let (first, draws, _) = pair(&mut h, true);
    let settled = draws.get();

    let step = Px::STEP.to_f32();
    for part in [0.1, 0.2, 0.3] {
        h.rsc[first].size.x = LayoutLen::px(100.0 + step * part);
        h.frame();
        assert_eq!(draws.get(), settled);
    }

    h.rsc[first].size.x = LayoutLen::px(100.0 + step);
    h.frame();
    assert_eq!(draws.get(), settled + 1);
}

#[test]
fn reporting_the_same_output_size_does_not_start_a_resize() {
    let mut h = Harness::new((400, 200));
    let draws = Rc::new(Cell::new(0));
    let leaf = ReadsBox {
        draws: draws.clone(),
    }
    .add(&mut h.rsc);
    h.set_root(leaf);
    let settled = draws.get();

    h.resize((400, 200));

    assert!(!h.needs_redraw());
    h.frame();
    assert_eq!(draws.get(), settled);
}

#[test]
fn narrowing_the_output_reflows_text_and_relays_out_around_it() {
    let mut h = Harness::new((600, 400));
    let para = wtext(
        "Wrapping shapes one source into as many lines as its container leaves \
        room for, so the height of a paragraph is an answer rather than a setting.",
    )
    .size(20)
    .wrap(true)
    .add(&mut h.rsc);
    let below = rect(Color::RED).add(&mut h.rsc);
    h.set_root((para, below).span(Dir::DOWN));
    let top = h.region(&below).expect("drew nothing").top_left.y;

    h.resize((300, 400));
    h.frame();

    let lower = h.region(&below).expect("drew nothing").top_left.y;
    assert!(lower > top, "same words, half the width: {top} -> {lower}");
}

#[test]
fn a_change_two_levels_under_its_reader_still_reaches_it() {
    let mut h = Harness::new((400, 400));
    // Every wrapper up to the outer pad read the size below it, so the outer
    // pad is what draws again -- and the span it hands the box to is the same
    // size as before, which is what lets a draw reuse its way past the leaf.
    let (leaf, _) = counted(&mut h, Size::px((100, 100).into()), true);
    let padded = leaf.pad(10).add(&mut h.rsc);
    let below = rect(Color::RED).add(&mut h.rsc);
    h.set_root((padded, below).span(Dir::DOWN).pad(12));
    assert_corners!(h, below, (12, 132), (388, 388));

    h.rsc[leaf].size = Size::px((100, 200).into());
    h.frame();

    assert_corners!(h, below, (12, 232), (388, 388));
}

/// Reads nothing of its box, so its drawing holds for any length, and has a
/// child so that whatever asks about the subtree has one to reach.
struct Stretchy {
    inner: StrongWidget,
    draws: Rc<Cell<usize>>,
}

impl Widget for Stretchy {
    fn draw(&mut self, painter: &mut Painter) -> Size {
        self.draws.set(self.draws.get() + 1);
        painter.widget(&self.inner).size()
    }
}

#[test]
fn stretching_a_subtree_carries_the_children_in_it() {
    let mut h = Harness::new((400, 400));
    let first = rect(Color::RED).height(40).add(&mut h.rsc);
    let inner = rect(Color::BLUE).add(&mut h.rsc);
    let draws = Rc::new(Cell::new(0));
    let outer = Stretchy {
        inner: inner.add_strong(&mut h.rsc),
        draws: draws.clone(),
    }
    .add(&mut h.rsc);
    h.set_root((first, outer).span(Dir::DOWN));
    let settled = draws.get();
    assert_corners!(h, inner, (0, 40), (400, 400));

    h.set_len(first, Axis::Y, 80);
    h.frame();

    assert_eq!(
        draws.get(),
        settled,
        "its drawing follows its box, rather than being made again"
    );
    assert_corners!(h, outer, (0, 80), (400, 400));
    assert_corners!(h, inner, (0, 80), (400, 400));
}

#[test]
fn a_widened_row_redraws_what_reads_its_length_and_nothing_else() {
    let mut h = Harness::new((400, 200));
    // What a transcript row is: something whose shaping depends on the width
    // it is given, beside something that only has to be the right shape.
    let (wraps, wrap_draws) = counted(&mut h, Size::LEFTOVER, true);
    let (backing, back_draws) = counted(&mut h, Size::LEFTOVER, false);
    let row = (backing, wraps).span(Dir::RIGHT).add(&mut h.rsc);
    let bar = rect(Color::RED).width(100).add(&mut h.rsc);
    h.set_root((bar, row).span(Dir::RIGHT));
    let (settled_wrap, settled_back) = (wrap_draws.get(), back_draws.get());

    h.set_len(bar, Axis::X, 200);
    h.frame();

    // The span reads every child's size, so redrawing one takes the span
    // with it -- and the span then measures and places the redrawn child.
    assert!(wrap_draws.get() > settled_wrap, "reads the width it got");
    assert_eq!(back_draws.get(), settled_back, "only has to be the shape");
    assert_corners!(h, backing, (200, 0), (300, 200));
    assert_corners!(h, wraps, (300, 0), (400, 200));
}

#[test]
fn a_declared_length_child_is_not_redrawn_when_the_box_around_it_grows() {
    let mut h = Harness::new((400, 200));
    // Its box is a fixed 80 wherever the row's edges end up, so drawing it
    // again would be for a width it does not have. The declared width is what
    // lets the span say that without drawing it: a width the span learnt by
    // drawing the child in its own box is only an answer for that box.
    let (counter, draws) = counted(&mut h, Size::from((80, 200)), true);
    let fixed = counter.width(80).add(&mut h.rsc);
    let (leftover, _) = counted(&mut h, Size::LEFTOVER, false);
    let row = (fixed, leftover).span(Dir::RIGHT).add(&mut h.rsc);
    let bar = rect(Color::RED).width(100).add(&mut h.rsc);
    h.set_root((bar, row).span(Dir::RIGHT));
    let settled = draws.get();

    h.set_len(bar, Axis::X, 200);
    h.frame();

    assert_eq!(draws.get(), settled, "its own length did not change");
    assert_corners!(h, fixed, (200, 0), (280, 200));
}

/// A retained drawing belongs to the layer it was made on: asked for again
/// on another one it has to be drawn there, since nothing about its geometry
/// says it is in a list that paints at a different moment.
#[test]
fn a_widget_asked_again_on_another_layer_is_drawn_there() {
    /// Draws its child on its own layer, then again one layer in -- which is
    /// what a container measuring a child by drawing it used to do.
    struct Twice(StrongWidget);

    impl Widget for Twice {
        fn draw(&mut self, painter: &mut Painter) -> Size {
            let size = painter.widget(&self.0).size();
            painter.child_layer();
            painter.widget(&self.0);
            size
        }
    }

    let mut h = Harness::new((400, 200));
    let (front, draws) = counted(&mut h, Size::from((100, 50)), false);
    let outer = Twice(front.add_strong(&mut h.rsc)).add(&mut h.rsc);
    h.set_root(outer);
    h.frame();

    assert_ne!(
        h.render.active[&front.id()].layer,
        h.render.active[&outer.id()].layer,
        "the first drawing was kept, on the layer it was measured on"
    );
    assert_eq!(draws.get(), 2, "the second ask could not reuse the first");
}

/// Which is why `Stack` measures the child that sizes it on the layer that
/// child draws on: one drawing, above the background it stacks over, rather
/// than one on each layer and the wrong one kept.
#[test]
fn a_stacks_sizing_child_is_drawn_once_where_it_belongs() {
    let mut h = Harness::new((400, 200));
    let background = rect(Color::RED).add(&mut h.rsc);
    let (front, draws) = counted(&mut h, Size::from((100, 50)), false);
    let stack = Stack {
        children: vec![
            background.add_strong(&mut h.rsc),
            front.add_strong(&mut h.rsc),
        ],
        size: StackSize::Child(1),
    }
    .add(&mut h.rsc);
    h.set_root(stack);
    h.frame();

    let layer = |id| h.render.active[&id].layer;
    assert_ne!(layer(front.id()), layer(stack.id()));
    assert_ne!(layer(front.id()), layer(background.id()));
    assert_eq!(draws.get(), 1);
}

/// A widget's own mask is not the one it inherited, and a redraw of it
/// inherits the second: handing back the first is handing it its own mask to
/// set a second time, which `set_mask` asserts against.
#[test]
fn a_masked_widget_redrawn_on_its_own_sets_its_mask_again() {
    let mut h = Harness::new((400, 200));
    let inner = rect(Color::BLUE).add(&mut h.rsc);
    let masked = inner.masked().add(&mut h.rsc);
    let other = rect(Color::RED).width(100).add(&mut h.rsc);
    h.set_root((other, masked).span(Dir::RIGHT));
    h.rsc.widgets_mut().get_dyn_mut(masked.id());
    h.frame();
    assert_corners!(h, inner, (100, 0), (400, 200));
}

/// The two spans a subtree changes hands between, and the branch that is not
/// in the tree yet -- kept alive by the test until it is.
struct Handover {
    leaf: WidgetId,
    first: WeakWidget<Span>,
    second: WeakWidget<Span>,
    root: WeakWidget<Span>,
    spare: StrongWidget,
}

/// A subtree that changes hands while its box does not move, so nothing about
/// reusing its drawing says it changed parents. `deeper` puts a span between
/// the root and `second`, so it changes depth by changing hands as well.
fn plant_handover(h: &mut Harness, moved: bool, deeper: bool, width: f32) -> Handover {
    let leaf = rect(Color::RED).add(&mut h.rsc);
    let sized = leaf.width(width).add(&mut h.rsc);
    let holder = (sized,).span(Dir::RIGHT).add(&mut h.rsc);
    let first = Span {
        children: match moved {
            true => Vec::new(),
            false => vec![holder.add_strong(&mut h.rsc)],
        },
        dir: Dir::RIGHT,
        gap: Px::ZERO,
    }
    .add(&mut h.rsc);
    let second = Span {
        children: match moved {
            true => vec![holder.add_strong(&mut h.rsc)],
            false => Vec::new(),
        },
        dir: Dir::RIGHT,
        gap: Px::ZERO,
    }
    .add(&mut h.rsc);
    let branch = match deeper {
        true => (second,).span(Dir::RIGHT).add_strong(&mut h.rsc),
        false => second.add_strong(&mut h.rsc),
    };
    let (in_tree, spare) = match moved {
        true => (branch, first.add_strong(&mut h.rsc)),
        false => (first.add_strong(&mut h.rsc), branch),
    };
    let root = Span {
        children: vec![in_tree],
        dir: Dir::RIGHT,
        gap: Px::ZERO,
    }
    .add(&mut h.rsc);
    h.state.root = Some(root.add_strong(&mut h.rsc));
    Handover {
        leaf: sized.id(),
        first,
        second,
        root,
        spare,
    }
}

/// Moves the subtree and swaps the branch it sits in for the one it left.
fn hand_over(h: &mut Harness, tree: Handover) -> WidgetId {
    let holder = h.rsc[tree.first].children.remove(0);
    h.rsc[tree.second].children.push(holder);
    h.rsc[tree.root].children.clear();
    h.rsc[tree.root].children.push(tree.spare);
    h.frame();
    tree.leaf
}

#[test]
fn a_subtree_that_changed_parents_is_not_undrawn_by_the_one_it_left() {
    let mut warm = Harness::new((400, 200));
    let tree = plant_handover(&mut warm, false, false, 40.0);
    warm.frame();
    let leaf = hand_over(&mut warm, tree);

    let mut cold = Harness::new((400, 200));
    let grown = plant_handover(&mut cold, true, false, 40.0);
    cold.frame();

    assert_eq!(
        warm.region(&leaf),
        cold.region(&grown.leaf),
        "the span it left still listed it and undrew it"
    );
}

#[test]
fn a_subtree_that_changed_parents_settles_at_the_depth_it_moved_to() {
    let mut warm = Harness::new((400, 200));
    let tree = plant_handover(&mut warm, false, true, 40.0);
    warm.frame();
    let leaf = hand_over(&mut warm, tree);
    // After it has changed hands, so what has to reach the new parent is a
    // change made under the subtree it now holds.
    warm.set_len(leaf, Axis::X, LayoutLen::px(90.0));
    warm.frame();

    let mut cold = Harness::new((400, 200));
    let grown = plant_handover(&mut cold, true, true, 90.0);
    cold.frame();

    assert_eq!(
        warm.region(&leaf),
        cold.region(&grown.leaf),
        "the span it moved to is the one the change has to reach"
    );
}