diff --git a/src/widget/position/set_size.rs b/src/widget/position/set_size.rs
index ffb4e52..8ec989e 100644
--- a/src/widget/position/set_size.rs
+++ b/src/widget/position/set_size.rs
@@ -8,7 +8,20 @@ pub struct SetSize {
 
 impl Widget for SetSize {
     fn draw(&mut self, painter: &mut Painter) -> Size {
-        let child = painter.widget(&self.inner).size();
+        // A declared length is what the child gets, whatever box this widget
+        // was offered before its parent knew that. Measuring it anywhere else
+        // asks about a box it will not have, and the answer on the other axis
+        // is taken under that: a wrapping text measured in the whole width
+        // reports one line, and nothing revisits it once the real width
+        // arrives.
+        let mut region = UiRegion::FULL;
+        for (axis, len) in [(Axis::X, self.x), (Axis::Y, self.y)] {
+            if let Some(len) = len {
+                let span = region.axis_mut(axis);
+                span.end = span.start + len.apply_rest();
+            }
+        }
+        let child = painter.widget_within(&self.inner, region).size();
         Size {
             x: self.x.unwrap_or(child.x),
             y: self.y.unwrap_or(child.y),
diff --git a/tests/trace_unsettled.rs b/tests/trace_unsettled.rs
new file mode 100644
index 0000000..ea61ef6
--- /dev/null
+++ b/tests/trace_unsettled.rs
@@ -0,0 +1,97 @@
+//! Traces the six-widget tree in `unsettled.rs`, to see what box its text is
+//! actually drawn in on a first frame against a settled one.
+
+#![cfg(feature = "layout-diagnostics")]
+
+use iris::core::layout_diagnostics::{self as diag, TraceEvent};
+use iris::harness::Harness;
+use iris::prelude::*;
+
+fn plant(h: &mut Harness) -> Vec<WidgetId> {
+    let plain = wtext("Wrapping").size(16).wrap(false).add(&mut h.rsc);
+    let wrapped = wtext("Wrapping shapes").size(16).wrap(true).add(&mut h.rsc);
+    let sized = SetSize {
+        inner: wrapped.add_strong(&mut h.rsc),
+        x: Some(Len::px(76.0)),
+        y: None,
+    }
+    .add(&mut h.rsc);
+    let aligned = Aligned {
+        inner: sized.add_strong(&mut h.rsc),
+        align: Align {
+            x: Some(AxisAlign::Pos),
+            y: Some(AxisAlign::Pos),
+        },
+    }
+    .add(&mut h.rsc);
+    let stack = Stack {
+        children: vec![plain.add_strong(&mut h.rsc), aligned.add_strong(&mut h.rsc)],
+        size: StackSize::Child(0),
+    }
+    .add(&mut h.rsc);
+    let root = (stack,).span(Dir::RIGHT).add(&mut h.rsc);
+    h.state.root = Some(root.add_strong(&mut h.rsc));
+    vec![
+        plain.id(),
+        wrapped.id(),
+        sized.id(),
+        aligned.id(),
+        stack.id(),
+        root.id(),
+    ]
+}
+
+fn dump(label: &str, report: &diag::Report, text: WidgetId) {
+    println!("--- {label} ---");
+    for event in report.traces() {
+        match event {
+            TraceEvent::DrawRequest {
+                id,
+                region,
+                pixel_size,
+                ..
+            } if *id == text => {
+                println!(
+                    "  draw in {:.2}x{:.2}  region {region:?}",
+                    pixel_size.x, pixel_size.y
+                )
+            }
+            TraceEvent::SizeReported { id, size } if *id == text => {
+                println!("  reported {size}")
+            }
+            TraceEvent::SizeRead { id, reader, size } if *id == text => {
+                println!("  size read by {reader:?}: {size}")
+            }
+            TraceEvent::Placed { id, parent, region } if *id == text => {
+                println!("  placed by {parent:?} at {region:?}")
+            }
+            TraceEvent::Reuse { id, outcome } if *id == text => println!("  reuse: {outcome:?}"),
+            _ => {}
+        }
+    }
+}
+
+#[test]
+#[ignore = "a diagnostic, not a check"]
+fn what_box_the_text_is_drawn_in() {
+    diag::clear_traced_widgets();
+    let _ = diag::take();
+    let mut h = Harness::new((640, 900));
+    let ids = plant(&mut h);
+    let text = ids[1];
+    diag::trace_widget(text);
+    let _ = diag::take();
+
+    h.frame();
+    dump("first frame", &diag::take(), text);
+
+    for _ in 0..2 {
+        for &id in &ids {
+            h.rsc.widgets_mut().get_dyn_mut(id);
+        }
+        let _ = diag::take();
+        h.frame();
+        dump("repaint", &diag::take(), text);
+    }
+    diag::clear_traced_widgets();
+}