Constrain offered boxes with independent widget size bounds

tests/cases/plan.rs

@@ -31,8 +31,8 @@ fn some_edits(seed: u64, of: &Plan) -> Edits {
                 (
                     i,
                     SizeRules {
-                        x: SizeRule::Exact(LayoutLen::LEFTOVER),
-                        y: SizeRule::Free,
+                        x: SizeRule::from(LayoutLen::LEFTOVER),
+                        y: SizeRule::FREE,
                     },
                 )
             })