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layout: Skip box tree construction when possible (#37957)

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When a style change does not chang the structure of the box tree, it is
possible to skip box tree rebuilding for an element. This change adds
support for reusing old box trees when no element has that type of
damage. In order to make this happen, there needs to be a type of
"empty" `LayoutDamage` that just indicates that a fragment tree layout
is necessary.

This is the first step toward incremental fragment tree layout.

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

Signed-off-by: Martin Robinson <mrobinson@igalia.com>
Co-authored-by: Oriol Brufau <obrufau@igalia.com>
This commit is contained in:
Martin Robinson 2025-07-09 19:33:09 +02:00 • committed by GitHub
parent d5d131c172
commit 436c9072c4
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GPG key ID: B5690EEEBB952194
13 changed files with 181 additions and 78 deletions
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35
components/layout/traversal.rs
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@ -97,8 +97,9 @@ where
pub(crate) fn compute_damage_and_repair_style(
context: &SharedStyleContext,
node: ServoLayoutNode<'_>,
damage_from_environment: RestyleDamage,
) -> RestyleDamage {
compute_damage_and_repair_style_inner(context, node, RestyleDamage::empty())
compute_damage_and_repair_style_inner(context, node, damage_from_environment)
}
pub(crate) fn compute_damage_and_repair_style_inner(
@ -107,6 +108,7 @@ pub(crate) fn compute_damage_and_repair_style_inner(
damage_from_parent: RestyleDamage,
) -> RestyleDamage {
let mut element_damage;
let original_element_damage;
let element_data = &node
.style_data()
.expect("Should not run `compute_damage` before styling.")
@ -114,6 +116,7 @@ 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;
@ -134,10 +137,6 @@ pub(crate) fn compute_damage_and_repair_style_inner(
}
}
if element_damage != RestyleDamage::reconstruct() && !element_damage.is_empty() {
node.repair_style(context);
}
// If one of our children needed to be reconstructed, we need to recollect children
// during box tree construction.
if damage_from_children.contains(LayoutDamage::recollect_box_tree_children()) {
@ -145,11 +144,27 @@ pub(crate) fn compute_damage_and_repair_style_inner(
element_data.borrow_mut().damage.insert(element_damage);
}
if element_damage.contains(LayoutDamage::recollect_box_tree_children()) {
node.invalidate_cached_fragment();
}
// Only propagate up layout phases from children, as other types of damage are
// incorporated into `element_damage` above.
element_damage | (damage_from_children & RestyleDamage::RELAYOUT)
let damage_for_parent = element_damage | (damage_from_children & RestyleDamage::RELAYOUT);
// If we are going to potentially reuse this box tree node, then clear any cached
// fragment layout.
//
// TODO: If this node has `recollect_box_tree_children` damage, this is unecessary
// unless it's entirely above the dirty root.
if element_damage != RestyleDamage::reconstruct() &&
damage_for_parent.contains(RestyleDamage::RELAYOUT)
{
node.clear_fragment_layout_cache();
}
// 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() {
node.repair_style(context);
}
damage_for_parent
}