Measure container children without intermediate placement

tests/cases/retained.rs

@@ -914,3 +914,40 @@ fn resizing_a_fixed_frame_recomposes_its_contents_without_drawing_them() {
         assert_eq!(mask(&warm, leaf.id()), mask(&cold, other.id()));
     }
 }
+
+#[test]
+fn a_span_does_not_place_its_measurement_before_assigning_the_childs_slot() {
+    struct MeasuredBox(Rc<Cell<usize>>);
+
+    impl Widget for MeasuredBox {
+        fn draw(&mut self, painter: &mut Painter) -> Size {
+            self.0.set(self.0.get() + 1);
+            painter.px_size();
+            painter.primitive(RectPrimitive::color(Color::BLUE));
+            Size::from((100, 50))
+        }
+    }
+
+    let mut h = Harness::new((400, 200));
+    let draws = Rc::new(Cell::new(0));
+    let leaf = MeasuredBox(draws.clone()).add(&mut h.rsc);
+    h.set_root((leaf,).span(Dir::RIGHT).width(rel(1.0)).height(rel(1.0)));
+
+    assert_eq!(draws.get(), 3);
+    assert_corners!(h, leaf, (0, 75), (100, 125));
+    assert_eq!(
+        primitive_bounds(&h, leaf.id()),
+        vec![h.region(&leaf.id()).unwrap()]
+    );
+    h.frame();
+    assert_eq!(draws.get(), 3);
+
+    h.resize((600, 300));
+    h.frame();
+    assert_eq!(draws.get(), 6);
+    assert_corners!(h, leaf, (0, 125), (100, 175));
+    assert_eq!(
+        primitive_bounds(&h, leaf.id()),
+        vec![h.region(&leaf.id()).unwrap()]
+    );
+}