Add explicit orthogonal span sizing

src/random.rs

@@ -308,6 +308,12 @@ impl<Rsc: UiRsc + 'static> Grow<'_, Rsc> {
             children,
             dir,
             gap: self.rng.below(3) as f32 * 4.0,
+            // Derive this from an existing choice: a seed must keep growing
+            // the same tree when the generator gains another configuration.
+            ortho: match dir.axis {
+                Axis::X => OrthoSize::Full,
+                Axis::Y => OrthoSize::Children,
+            },
         }
         .add(self.rsc);
         self.tree.ids.push(id.id());