diff --git a/src/random.rs b/src/random.rs
index 30700bd..ae3ad94 100644
--- a/src/random.rs
+++ b/src/random.rs
@@ -84,6 +84,35 @@ pub struct Tree {
     pub detached: Vec<StrongWidget>,
 }
 
+/// Branches on a child's measured length. Comparing boxes catches a widget
+/// that moved; this catches one that believed a measurement a cold start
+/// would not have given it, by turning that into a different tree. Its own
+/// configuration never changes, so which side draws is a property of the
+/// layout alone.
+pub struct Branch {
+    pub probe: StrongWidget,
+    pub wide: StrongWidget,
+    pub narrow: StrongWidget,
+    pub threshold: f32,
+}
+
+impl Widget for Branch {
+    fn draw(&mut self, painter: &mut Painter) -> Size {
+        let mut top = UiRegion::FULL;
+        top.y.end = top.y.start.offset(40.0);
+        let measured = painter.place(&self.probe, top).len(Axis::X);
+        let px = measured.apply_rest().to_px(painter.px_len(Axis::X));
+
+        let mut rest = UiRegion::FULL;
+        rest.y.start = rest.y.start.offset(40.0);
+        match px > self.threshold {
+            true => painter.place(&self.wide, rest),
+            false => painter.place(&self.narrow, rest),
+        };
+        Size::REST
+    }
+}
+
 pub struct Spanned {
     pub id: WeakWidget<Span>,
     /// Leaves grown with the span whether or not they end up in it, so both
@@ -199,6 +228,23 @@ impl<Rsc: UiRsc + 'static> Grow<'_, Rsc> {
             self.tree.ids.push(id.id());
             return id.add_strong(self.rsc);
         }
+        if positioned == 2 {
+            // Both sides are grown either way, so a tree that draws one has
+            // the same ids as a tree that draws the other.
+            let probe = self.node(depth - 1);
+            let wide = self.node(depth - 1);
+            let narrow = self.node(depth - 1);
+            let threshold = self.rng.below(500) as f32;
+            let id = Branch {
+                probe,
+                wide,
+                narrow,
+                threshold,
+            }
+            .add(self.rsc);
+            self.tree.ids.push(id.id());
+            return id.add_strong(self.rsc);
+        }
         if positioned == 1 {
             let inner = self.node(depth - 1);
             let inner = self.sized(inner);
diff --git a/tests/determinism.rs b/tests/determinism.rs
new file mode 100644
index 0000000..da9ddf6
--- /dev/null
+++ b/tests/determinism.rs
@@ -0,0 +1,101 @@
+//! A measurement that decides control flow.
+//!
+//! Comparing boxes catches a widget that moved. It does not catch a widget
+//! that measured a child, believed a different answer from the one a cold
+//! start would give, and took the other branch -- which is the same defect
+//! arriving somewhere it cannot be ignored. A widget here branches on what it
+//! measured, so a disagreement shows up as a different tree.
+
+use iris::harness::Harness;
+use iris::prelude::*;
+
+/// Measures `probe` across `axis` and draws one of two children on the
+/// answer. Its own configuration never changes, so which child is drawn is a
+/// property of the layout alone.
+struct BranchesOnMeasurement {
+    probe: StrongWidget,
+    wide: StrongWidget,
+    narrow: StrongWidget,
+    threshold: f32,
+}
+
+impl Widget for BranchesOnMeasurement {
+    fn draw(&mut self, painter: &mut Painter) -> Size {
+        let mut top = UiRegion::FULL;
+        top.y.end = top.y.start.offset(40.0);
+        let measured = painter.place(&self.probe, top).len(Axis::X);
+        let px = measured.apply_rest().to_px(painter.px_len(Axis::X));
+
+        let mut rest = UiRegion::FULL;
+        rest.y.start = rest.y.start.offset(40.0);
+        match px > self.threshold {
+            true => painter.place(&self.wide, rest),
+            false => painter.place(&self.narrow, rest),
+        };
+        Size::REST
+    }
+}
+
+fn plant(h: &mut Harness, threshold: f32) -> (WidgetId, WidgetId) {
+    let words = "the quick brown fox jumps over the lazy dog and keeps running";
+    let probe = wtext(words).size(16).wrap(true).add(&mut h.rsc);
+    let wide = rect(Color::RED).add(&mut h.rsc);
+    let narrow = rect(Color::BLUE).add(&mut h.rsc);
+    let branch = BranchesOnMeasurement {
+        probe: probe.add_strong(&mut h.rsc),
+        wide: wide.add_strong(&mut h.rsc),
+        narrow: narrow.add_strong(&mut h.rsc),
+        threshold,
+    }
+    .add(&mut h.rsc);
+    let side = rect(Color::GREEN).width(120).add(&mut h.rsc);
+    h.set_root((side, branch).span(Dir::RIGHT));
+    (wide.id(), narrow.id())
+}
+
+/// Which of the two branches drew, as a pair a test can compare.
+fn taken(h: &Harness, wide: WidgetId, narrow: WidgetId) -> (bool, bool) {
+    (h.region(&wide).is_some(), h.region(&narrow).is_some())
+}
+
+#[test]
+fn a_branch_taken_on_a_measurement_holds_across_repaints() {
+    for threshold in [0.0, 200.0, 400.0, 600.0, 779.0, 780.0, 781.0, 2000.0] {
+        let mut h = Harness::new((900, 600));
+        let (wide, narrow) = plant(&mut h, threshold);
+        let first = taken(&h, wide, narrow);
+        assert_ne!(first, (false, false), "threshold {threshold}: neither drew");
+
+        for frame in 0..4 {
+            h.rsc.widgets_mut().get_dyn_mut(wide);
+            h.rsc.widgets_mut().get_dyn_mut(narrow);
+            h.frame();
+            assert_eq!(
+                taken(&h, wide, narrow),
+                first,
+                "threshold {threshold}, repaint {frame}: the branch moved when nothing did"
+            );
+        }
+    }
+}
+
+#[test]
+fn a_branch_taken_on_a_measurement_is_the_one_a_cold_start_takes() {
+    for threshold in [0.0, 200.0, 400.0, 600.0, 779.0, 780.0, 781.0, 2000.0] {
+        let mut warm = Harness::new((900, 600));
+        let (wide, narrow) = plant(&mut warm, threshold);
+        warm.resize((640, 480));
+        warm.frame();
+        warm.rsc.widgets_mut().get_dyn_mut(wide);
+        warm.frame();
+
+        let mut cold = Harness::new((640, 480));
+        let (cwide, cnarrow) = plant(&mut cold, threshold);
+
+        assert_eq!(
+            taken(&warm, wide, narrow),
+            taken(&cold, cwide, cnarrow),
+            "threshold {threshold}: warm and cold took different branches"
+        );
+    }
+}
diff --git a/tests/generated.rs b/tests/generated.rs
index 124018d..e77f99e 100644
--- a/tests/generated.rs
+++ b/tests/generated.rs
@@ -224,8 +224,15 @@ fn reshuffled(seed: u64, shuffle: Shuffle) {
     let mut warm = Harness::new((900, 1200));
     let mut grown = plant(&mut warm, seed, &Edits::default());
     // Some seeds grow nothing but wrappers, and a shuffle with no span to
-    // shuffle is not the same thing as one that had no effect.
-    if grown.spans.is_empty() {
+    // shuffle is not the same thing as one that had no effect. A span behind
+    // a branch nobody took is the same kind of nothing: it is not drawn, so
+    // shuffling it cannot move anything.
+    let shuffles = grown
+        .spans
+        .iter()
+        .step_by(3)
+        .any(|span| warm.region(&span.id.id()).is_some());
+    if !shuffles {
         return;
     }
     let before: Vec<_> = grown.ids.iter().map(|id| warm.region(id)).collect();