tor-browser

commit 6cb49e422cad3e0bba008296df7d5929b25e8b43
parent 04b013b47241e496684d5f7eb4368c0898166d87
Author: Denis Palmeiro <dpalmeiro@mozilla.com>
Date:   Tue, 30 Dec 2025 15:43:10 +0000

Bug 2006172: Allow style sharing across shadow hosts. r=emilio,firefox-style-system-reviewers

Allow shadow hosts to share styles when they have the same CascadeData pointer,
ensuring they have the same :host rules. Improves TodoMVC-WebComponents by ~10%.

Differential Revision: https://phabricator.services.mozilla.com/D277357

Diffstat:
M
servo/components/style/sharing/mod.rs
 | 
27
+++++++++++++--------------

1 file changed, 13 insertions(+), 14 deletions(-)
diff --git a/servo/components/style/sharing/mod.rs b/servo/components/style/sharing/mod.rs
@@ -68,7 +68,7 @@ use crate::applicable_declarations::ApplicableDeclarationBlock;
 use crate::bloom::StyleBloom;
 use crate::computed_value_flags::ComputedValueFlags;
 use crate::context::{SharedStyleContext, StyleContext};
-use crate::dom::{SendElement, TElement};
+use crate::dom::{SendElement, TElement, TShadowRoot};
 use crate::properties::ComputedValues;
 use crate::rule_tree::StrongRuleNode;
 use crate::selector_map::RelevantAttributes;
@@ -654,16 +654,6 @@ impl<E: TElement> StyleSharingCache<E> {
             return;
         }
 
-        // We can't share style across shadow hosts right now, because they may
-        // match different :host rules.
-        //
-        // TODO(emilio): We could share across the ones that don't have :host
-        // rules or have the same.
-        if element.shadow_root().is_some() {
-            debug!("Failing to insert into the cache: Shadow Host");
-            return;
-        }
-
         // If the element has running animations, we can't share style.
         //
         // This is distinct from the specifies_{animations,transitions} check below,
@@ -817,9 +807,18 @@ impl<E: TElement> StyleSharingCache<E> {
             return None;
         }
 
-        if target.element.shadow_root().is_some() {
-            trace!("Miss: Shadow host");
-            return None;
+        // Shadow hosts can share style when they have matching CascadeData pointers, which
+        // ensures they match the same :host rules.
+        match (
+            target.element.shadow_root().and_then(|s| s.style_data()),
+            candidate.element.shadow_root().and_then(|s| s.style_data()),
+        ) {
+            (Some(td), Some(cd)) if std::ptr::eq(td, cd) => {},
+            (None, None) => {},
+            _ => {
+                trace!("Miss: Different shadow root style data");
+                return None;
+            },
         }
 
         if target.element.has_animations(shared_context)