layout: optimize the propagation and cleanup of RestyleDamage (#38199)

This change optimizes `RestyleDamage` propagation and cleanup to reduce
unnecessary box reconstruction and relayouts, while preventing damage
from the current reflow affecting subsequent ones. Improvements include:
- For box damage caused by `NodeDamage`, `RestyleDamage::RELAYOUT` is no
longer marked immediately—avoiding erroneous propagation of
`LayoutDamage::RECOLLECT_BOX_TREE_CHILDREN` to descendants during
`RestyleDamage` propagation.
- Clearing damage for nodes whose boxes will be preserved, preventing it
from carrying over to the next reflow and increasing its workload.

Testing: This should not change observable behavior and is thus covered
by existing WPT tests. Although Servo lacks performance test cases,
manual testing shows that this modification reduces reflow time by
nearly 250ms, representing a decrease of approximately 25%.

Signed-off-by: sharpshooter_pt <ibluegalaxy_taoj@163.com>

components/layout/traversal.rs

@@ -2,6 +2,7 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
 
+use bitflags::Flags;
 use layout_api::LayoutDamage;
 use layout_api::wrapper_traits::LayoutNode;
 use script::layout_dom::ServoLayoutNode;
@@ -117,18 +118,23 @@ pub(crate) fn compute_damage_and_repair_style_inner(
     {
         let mut element_data = element_data.borrow_mut();
         original_element_damage = element_data.damage;
-        element_data.damage.insert(damage_from_parent);
-        element_damage = element_data.damage;
+        element_damage = original_element_damage | damage_from_parent;
 
         if let Some(ref style) = element_data.styles.primary {
             if style.get_box().display == Display::None {
+                element_data.damage = element_damage;
                 return element_damage;
             }
         }
     }
 
     // If we are reconstructing this node, then all of the children should be reconstructed as well.
-    let damage_for_children = element_damage | damage_from_parent;
+    // Otherwise, do not propagate down its box damage.
+    let mut damage_for_children = element_damage;
+    if !element_damage.contains(LayoutDamage::rebuild_box_tree()) {
+        damage_for_children.truncate();
+    }
+
     let mut damage_from_children = RestyleDamage::empty();
     for child in iter_child_nodes(node) {
         if child.is_element() {
@@ -141,7 +147,12 @@ pub(crate) fn compute_damage_and_repair_style_inner(
     // during box tree construction.
     if damage_from_children.contains(LayoutDamage::recollect_box_tree_children()) {
         element_damage.insert(LayoutDamage::recollect_box_tree_children());
-        element_data.borrow_mut().damage.insert(element_damage);
+    }
+
+    // If this node's box will not be preserved, we need to relayout its box tree.
+    let element_layout_damage = LayoutDamage::from(element_damage);
+    if element_layout_damage.has_box_damage() {
+        element_damage.insert(RestyleDamage::RELAYOUT);
     }
 
     // Only propagate up layout phases from children, as other types of damage are
@@ -160,11 +171,19 @@ pub(crate) fn compute_damage_and_repair_style_inner(
     }
 
     // If the box will be preserved, update the box's style and also in any fragments
-    // that haven't been cleared.
-    let element_layout_damage: LayoutDamage = element_damage.into();
-    if !element_layout_damage.has_box_damage() && !original_element_damage.is_empty() {
+    // that haven't been cleared. Meanwhile, clear the damage to avoid affecting the
+    // next reflow.
+    if !element_layout_damage.has_box_damage() {
+        if !original_element_damage.is_empty() {
             node.repair_style(context);
         }
 
+        element_damage = RestyleDamage::empty();
+    }
+
+    if element_damage != original_element_damage {
+        element_data.borrow_mut().damage = element_damage;
+    }
+
     damage_for_parent
 }

components/shared/layout/layout_damage.rs

@@ -21,12 +21,11 @@ bitflags! {
 
 impl LayoutDamage {
     pub fn recollect_box_tree_children() -> RestyleDamage {
-        RestyleDamage::from_bits_retain(LayoutDamage::RECOLLECT_BOX_TREE_CHILDREN.bits()) |
-            RestyleDamage::RELAYOUT
+        RestyleDamage::from_bits_retain(LayoutDamage::RECOLLECT_BOX_TREE_CHILDREN.bits())
     }
 
     pub fn rebuild_box_tree() -> RestyleDamage {
-        RestyleDamage::from_bits_retain(LayoutDamage::REBUILD_BOX.bits()) | RestyleDamage::RELAYOUT
+        RestyleDamage::from_bits_retain(LayoutDamage::REBUILD_BOX.bits())
     }
 
     pub fn has_box_damage(&self) -> bool {