layout: Add a first pass at incremental box tree construction (#37751)

This change:

- Adds a new type of LayoutDamage that signifies that a box needs its
  children recollected, because one or more of them need to be rebuilt.
- During restyle damage propagation, propagate this new damage upward in
  the tree. Then box tree construction should be able to preserve any
  still-valid box tree nodes from box slots.
- During BlockLevelBox job finalization, if a box slot is valid and
  there is not LayoutDamage to the element, use the old box slot,
  ensuring that its fragment cache is invalidated.

Testing: This should not change observable behavior and thus is covered
by existing WPT tests.

Signed-off-by: Martin Robinson <mrobinson@igalia.com>
Co-authored-by: Oriol Brufau <obrufau@igalia.com>
Co-authored-by: coding-joedow <ibluegalaxy_taoj@163.com>

components/script/dom/document.rs

@@ -936,7 +936,7 @@ impl Document {
 
         // FIXME(emilio): This is very inefficient, ideally the flag above would
         // be enough and incremental layout could figure out from there.
-        node.dirty(NodeDamage::Other);
+        node.dirty(NodeDamage::ContentOrHeritage);
     }
 
     /// Remove any existing association between the provided id and any elements in this document.

components/script/dom/element.rs

@@ -23,6 +23,7 @@ use html5ever::{LocalName, Namespace, Prefix, QualName, local_name, namespace_pr
 use js::jsapi::Heap;
 use js::jsval::JSVal;
 use js::rust::HandleObject;
+use layout_api::LayoutDamage;
 use net_traits::ReferrerPolicy;
 use net_traits::request::CorsSettings;
 use selectors::Element as SelectorsElement;
@@ -358,9 +359,18 @@ impl Element {
         // NodeStyleDamaged, but I'm preserving existing behavior.
         restyle.hint.insert(RestyleHint::RESTYLE_SELF);
 
-        if damage == NodeDamage::Other {
-            doc.note_node_with_dirty_descendants(self.upcast());
-            restyle.damage = RestyleDamage::reconstruct();
+        match damage {
+            NodeDamage::Style => {},
+            NodeDamage::ContentOrHeritage => {
+                doc.note_node_with_dirty_descendants(self.upcast());
+                restyle
+                    .damage
+                    .insert(LayoutDamage::recollect_box_tree_children());
+            },
+            NodeDamage::Other => {
+                doc.note_node_with_dirty_descendants(self.upcast());
+                restyle.damage.insert(RestyleDamage::reconstruct());
+            },
         }
     }
 

components/script/dom/node.rs

@@ -3927,6 +3927,9 @@ impl VirtualMethods for Node {
 pub(crate) enum NodeDamage {
     /// The node's `style` attribute changed.
     Style,
+    /// The node's content or heritage changed, such as the addition or removal of
+    /// children.
+    ContentOrHeritage,
     /// Other parts of a node changed; attributes, text content, etc.
     Other,
 }