Bug 1618303 - Add an inherited style bit to know whether an element is in an opacity: 0 subtree. r=hiro

I think this should work for the animation throttling stuff.

Opacity works on the element tree, so I think this is sound.

Differential Revision: https://phabricator.services.mozilla.com/D64441

--HG--
extra : moz-landing-system : lando

layout/style/ComputedStyle.h

@@ -166,6 +166,10 @@ class ComputedStyle {
     return bool(Flags() & Flag::IS_ROOT_ELEMENT_STYLE);
   }
 
+  bool IsInOpacityZeroSubtree() const {
+    return bool(Flags() & Flag::IS_IN_OPACITY_ZERO_SUBTREE);
+  }
+
   ComputedStyle* GetCachedInheritingAnonBoxStyle(
       PseudoStyleType aPseudoType) const {
     MOZ_ASSERT(PseudoStyle::IsInheritingAnonBox(aPseudoType));

servo/components/style/properties/computed_value_flags.rs

@@ -67,6 +67,9 @@ bitflags! {
 
         /// Whether this style is the style of the document element.
         const IS_ROOT_ELEMENT_STYLE = 1 << 11;
+
+        /// Whether this element is inside an `opacity: 0` subtree.
+        const IS_IN_OPACITY_ZERO_SUBTREE = 1 << 12;
     }
 }
 
@@ -74,10 +77,11 @@ impl ComputedValueFlags {
     /// Flags that are unconditionally propagated to descendants.
     #[inline]
     fn inherited_flags() -> Self {
-        ComputedValueFlags::IS_RELEVANT_LINK_VISITED |
-            ComputedValueFlags::CAN_BE_FRAGMENTED |
-            ComputedValueFlags::IS_IN_PSEUDO_ELEMENT_SUBTREE |
-            ComputedValueFlags::HAS_TEXT_DECORATION_LINES
+        Self::IS_RELEVANT_LINK_VISITED |
+            Self::CAN_BE_FRAGMENTED |
+            Self::IS_IN_PSEUDO_ELEMENT_SUBTREE |
+            Self::HAS_TEXT_DECORATION_LINES |
+            Self::IS_IN_OPACITY_ZERO_SUBTREE
     }
 
     /// Flags that may be propagated to descendants.

servo/components/style/style_adjuster.rs

@@ -227,15 +227,21 @@ impl<'a, 'b: 'a> StyleAdjuster<'a, 'b> {
     fn set_bits(&mut self) {
         let display = self.style.get_box().clone_display();
 
-        if !display.is_contents() &&
-            !self
-                .style
-                .get_text()
-                .clone_text_decoration_line()
-                .is_empty()
-        {
-            self.style
-                .add_flags(ComputedValueFlags::HAS_TEXT_DECORATION_LINES);
+        if !display.is_contents() {
+            if !self
+                    .style
+                    .get_text()
+                    .clone_text_decoration_line()
+                    .is_empty()
+            {
+                self.style
+                    .add_flags(ComputedValueFlags::HAS_TEXT_DECORATION_LINES);
+            }
+
+            if self.style.get_effects().clone_opacity() == 0. {
+                self.style
+                    .add_flags(ComputedValueFlags::IS_IN_OPACITY_ZERO_SUBTREE);
+            }
         }
 
         if self.style.is_pseudo_element() {