feat: implement nested border-box layout and alignment

src/render/source_layout.rs

@@ -165,7 +165,11 @@ impl NativeCallable for Step {
                 Some(
                     plan.runtime
                         .measure_node(&node, children.clone(), constraints)
-                        .map_err(|_| invalid("native measurement exceeds layout limits"))?,
+                        .map_err(|error| {
+                            error.downcast_ref::<VmError>().cloned().unwrap_or_else(|| {
+                                invalid("native measurement exceeds layout limits")
+                            })
+                        })?,
                 )
             } else {
                 if !intrinsic {
@@ -211,8 +215,7 @@ impl NativeCallable for Step {
             if matches!(self.phase, Phase::Arrange) {
                 plan.runtime.arranged += 1;
                 plan.native_arrange[self.index] = intrinsic;
-                plan.custom_arrange[self.index] =
-                    !intrinsic || node.children().iter().any(|child| child.has_flex());
+                plan.custom_arrange[self.index] = !intrinsic || node.allocates_children();
             }
             (node, children, preferred, bounds, native_size)
         };
@@ -532,7 +535,13 @@ impl NativeCallable for Settle {
                     Size { width, height },
                 );
                 for child in children {
-                    let mut constraints = base;
+                    let natural = layout::occurrence_constraints(
+                        &plan.nodes[index],
+                        &plan.nodes[child],
+                        base,
+                        Size { width, height },
+                    );
+                    let mut constraints = natural;
                     if let Some(width) = plan.allocations[child][0] {
                         constraints[0] = width;
                         constraints[2] = width;
@@ -554,8 +563,8 @@ impl NativeCallable for Settle {
                                 // own replacement measurement. Source arrange
                                 // allocations remain authoritative.
                                 plan.allocations[child][main] = None;
-                                constraints[main] = base[main];
-                                constraints[main + 2] = base[main + 2];
+                                constraints[main] = natural[main];
+                                constraints[main + 2] = natural[main + 2];
                             }
                         }
                     }
@@ -844,7 +853,7 @@ pub fn start(
     let mut nodes = Vec::new();
     let mut work = vec![root.clone()];
     while let Some(node) = work.pop() {
-        custom |= node.has_flex();
+        custom |= node.has_flex() || node.allocates_children();
         work.extend(node.children().iter().rev().cloned());
         nodes.push(node);
     }
@@ -878,9 +887,12 @@ pub fn start(
     }
     let mut constraints = vec![runtime.root_constraints(); nodes.len()];
     for index in 0..nodes.len() {
-        let next = layout::child_constraints(&nodes[index], constraints[index]);
         for child in &children[index] {
-            constraints[*child] = next;
+            constraints[*child] = layout::initial_child_constraints(
+                &nodes[index],
+                &nodes[*child],
+                constraints[index],
+            );
         }
     }
     let count = nodes.len();