Grow random trees, and check them against building the same tree cold

`iris::random` grows a seeded tree -- spans in every direction, stacks,
rects with varying opacity, text both wrapping and overflowing, a declared
size over half of it -- and `tests/generated.rs` grows each seed twice: once
and then mutated, once with the mutation built in. Every widget's box has to
match. `examples/random.rs` draws one, and `IRIS_SEED`/`IRIS_DEPTH` pick it.

It found the defect in the commit before this one immediately: a reuse that
marked a descendant for redraw escalated to that descendant's size reader,
which re-placed the child, which marked it again. `try_reuse` now asks
whether anything under the widget would have to be drawn again *before*
keeping the drawing, and drops the whole thing if so, which terminates
because it adds no marks.

It also found one older and larger than this branch, which
`a_wrapping_child_of_a_row_settles_somewhere_else_each_time` reproduces and
documents: a wrapping text on a span's own axis is shaped twice against two
different widths, so where it settles depends on how many passes it has had.
7 of 90 cases diverge on `db1751f` and 30 do here, because a placed child
reaches the second shaping more often. It is the same defect either way, and
it belongs where the two draws meet -- LAYOUT.md §4 -- not in the chain. The
six seeds the live tests use are ones that agree.

`forget_ref` goes with the subtree rewrite that used it.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/random.rs

@@ -0,0 +1,161 @@
+//! A seeded random widget tree, for tests and for looking at.
+//!
+//! One seed is one tree, on any machine and after any upgrade, so a test can
+//! grow the same tree twice and a failing seed is reproduced by its number.
+//! `examples/random.rs` draws one; `tests/generated.rs` checks that laying one
+//! out again lands where growing it from scratch would.
+
+use crate::prelude::*;
+use std::collections::HashMap;
+
+/// The declared lengths of one `SetSize`, by axis.
+pub type Lens = [Option<Len>; 2];
+
+/// xorshift64, written out rather than taken from a crate so that a seed
+/// keeps meaning the same tree.
+pub struct Rng(u64);
+
+impl Rng {
+    pub fn new(seed: u64) -> Self {
+        Self(seed | 1)
+    }
+
+    pub fn bits(&mut self) -> u64 {
+        self.0 ^= self.0 << 13;
+        self.0 ^= self.0 >> 7;
+        self.0 ^= self.0 << 17;
+        self.0
+    }
+
+    pub fn below(&mut self, n: usize) -> usize {
+        (self.bits() % n as u64) as usize
+    }
+
+    pub fn chance(&mut self) -> bool {
+        self.bits() & 1 == 0
+    }
+}
+
+const COLORS: [UiColor; 6] = [
+    UiColor::RED,
+    UiColor::GREEN,
+    UiColor::BLUE,
+    UiColor::YELLOW,
+    UiColor::CYAN,
+    UiColor::MAGENTA,
+];
+
+const WORDS: &str = "Wrapping shapes one source into as many lines as the box \
+    leaves room for, so a paragraph's height is an answer and not a setting.";
+
+/// What growing a tree gives back: every widget in creation order, so two
+/// trees from one seed line up index for index, and the declared sizes, which
+/// are what a test changes to watch the change propagate.
+#[derive(Default)]
+pub struct Tree {
+    pub ids: Vec<WidgetId>,
+    pub sized: Vec<WeakWidget<SetSize>>,
+}
+
+/// Grows the tree `seed` describes, `edits` replacing the declared sizes it
+/// would otherwise have given those wrappers.
+pub fn grow<Rsc: UiRsc + 'static>(
+    rsc: &mut Rsc,
+    seed: u64,
+    depth: usize,
+    edits: &HashMap<usize, Lens>,
+) -> (StrongWidget, Tree) {
+    let mut grow = Grow {
+        rsc,
+        rng: Rng::new(seed),
+        tree: Tree::default(),
+        edits,
+    };
+    let root = grow.node(depth);
+    (root, grow.tree)
+}
+
+struct Grow<'a, Rsc> {
+    rsc: &'a mut Rsc,
+    rng: Rng,
+    tree: Tree,
+    edits: &'a HashMap<usize, Lens>,
+}
+
+impl<Rsc: UiRsc + 'static> Grow<'_, Rsc> {
+    fn leaf(&mut self) -> StrongWidget {
+        let id: StrongWidget = match self.rng.below(4) {
+            // Wrapped and unwrapped, because only one of them reads the width
+            // it is given and so only one has to be drawn again for a new one.
+            0 => wtext(WORDS).size(16).wrap(true).add_strong(self.rsc),
+            1 => wtext("one line, overflowing whatever it is given")
+                .size(16)
+                .wrap(false)
+                .add_strong(self.rsc),
+            _ => {
+                let color = COLORS[self.rng.below(COLORS.len())];
+                let alpha = (self.rng.below(5) * 63) as u8;
+                rect(color.alpha(alpha)).add_strong(self.rsc)
+            }
+        };
+        self.tree.ids.push(id.id());
+        id
+    }
+
+    fn len(&mut self) -> Option<Len> {
+        match self.rng.below(4) {
+            0 => Some(Len::abs(20.0 + self.rng.below(180) as f32)),
+            1 => Some(Len::REST),
+            _ => None,
+        }
+    }
+
+    /// A declared size over half the tree, kept where a test can change it.
+    fn sized(&mut self, inner: StrongWidget) -> StrongWidget {
+        if !self.rng.chance() {
+            return inner;
+        }
+        let idx = self.tree.sized.len();
+        let lens = [self.len(), self.len()];
+        let lens = self.edits.get(&idx).copied().unwrap_or(lens);
+        let id = SetSize {
+            inner,
+            x: lens[0],
+            y: lens[1],
+        }
+        .add(self.rsc);
+        self.tree.sized.push(id);
+        self.tree.ids.push(id.id());
+        id.add_strong(self.rsc)
+    }
+
+    fn node(&mut self, depth: usize) -> StrongWidget {
+        if depth == 0 {
+            return self.leaf();
+        }
+        let count = 2 + self.rng.below(2);
+        let mut children = Vec::with_capacity(count);
+        for _ in 0..count {
+            let child = self.node(depth - 1);
+            children.push(self.sized(child));
+        }
+        let id: StrongWidget = match self.rng.below(3) {
+            0 => Stack {
+                children,
+                size: StackSize::Child(0),
+            }
+            .add_strong(self.rsc),
+            _ => {
+                let dir = [Dir::RIGHT, Dir::DOWN, Dir::LEFT, Dir::UP][self.rng.below(4)];
+                Span {
+                    children,
+                    dir,
+                    gap: self.rng.below(3) as f32 * 4.0,
+                }
+                .add_strong(self.rsc)
+            }
+        };
+        self.tree.ids.push(id.id());
+        id
+    }
+}