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/shared/layout/layout_damage.rs

@@ -3,14 +3,29 @@
  * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
 
 use bitflags::bitflags;
+use style::selector_parser::RestyleDamage;
 
 bitflags! {
     /// Individual layout actions that may be necessary after restyling. This is an extension
     /// of `RestyleDamage` from stylo, which only uses the 4 lower bits.
-    #[derive(Clone, Copy, Debug, Eq, PartialEq)]
+    #[derive(Clone, Copy, Default, Eq, PartialEq)]
     pub struct LayoutDamage: u16 {
+        /// Recollect the box children for this element, because some of the them will be
+        /// rebuilt.
+        const RECOLLECT_BOX_TREE_CHILDREN = 0b011111111111 << 4;
         /// Rebuild the entire box for this element, which means that every part of layout
         /// needs to happena again.
         const REBUILD_BOX = 0b111111111111 << 4;
     }
 }
+
+impl LayoutDamage {
+    pub fn recollect_box_tree_children() -> RestyleDamage {
+        RestyleDamage::from_bits_retain(LayoutDamage::RECOLLECT_BOX_TREE_CHILDREN.bits()) |
+            RestyleDamage::RELAYOUT
+    }
+
+    pub fn has_box_damage(&self) -> bool {
+        self.intersects(Self::REBUILD_BOX)
+    }
+}