Constrain offered boxes with independent widget size bounds

tests/allocation_cost.rs

@@ -35,16 +35,16 @@ static ALLOCATOR: Counting = Counting;
 
 #[test]
 fn unchanged_tree_reuses_layout_storage() {
-    for deferred in [false, true] {
+    for rule in [
+        SizeRule::FREE,
+        leftover(1).clamp(20, 80).into(),
+        SizeRule::clamp(20.into(), 80.into()),
+    ] {
         let mut h = Harness::new((600, 200));
         let mut children: Vec<StrongWidget> = Vec::new();
         for _ in 0..8 {
             let a = rect(Color::RED).add(&mut h.rsc);
-            if deferred {
-                h.rsc
-                    .widgets_mut()
-                    .set_size_rule(a, Axis::X, leftover(1).clamp(20, 80).into());
-            }
+            h.rsc.widgets_mut().set_size_rule(a, Axis::X, rule.clone());
             let row = (a, rect(Color::BLUE))
                 .span(Dir::RIGHT)
                 .add_strong(&mut h.rsc);
@@ -74,7 +74,7 @@ fn unchanged_tree_reuses_layout_storage() {
             h.frame();
         }
         let allocations = COUNT.replace(None).unwrap();
-        println!("deferred={deferred}: {allocations} allocations over 100 resize frames");
+        println!("rule={rule:?}: {allocations} allocations over 100 resize frames");
         assert_eq!(allocations, 0);
     }
 }