style: Remove TNode::set_can_be_fragmented and TNode::can_be_fragmented.

Replace them instead by a computed value flag, the same way as the
IS_IN_DISPLAY_NONE_SUBTREE flag works.

components/style/dom.rs

@@ -239,13 +239,6 @@ pub trait TNode : Sized + Copy + Clone + Debug + NodeInfo + PartialEq {
 
     /// Get this node as a document, if it's one.
     fn as_document(&self) -> Option<Self::ConcreteDocument>;
-
-    /// Whether this node can be fragmented. This is used for multicol, and only
-    /// for Servo.
-    fn can_be_fragmented(&self) -> bool;
-
-    /// Set whether this node can be fragmented.
-    unsafe fn set_can_be_fragmented(&self, value: bool);
 }
 
 /// Wrapper to output the subtree rather than the single node when formatting

components/style/gecko/wrapper.rs

@@ -345,18 +345,6 @@ impl<'ln> TNode for GeckoNode<'ln> {
             None
         }
     }
-
-    #[inline]
-    fn can_be_fragmented(&self) -> bool {
-        // FIXME(SimonSapin): Servo uses this to implement CSS multicol / fragmentation
-        // Maybe this isn’t useful for Gecko?
-        false
-    }
-
-    unsafe fn set_can_be_fragmented(&self, _value: bool) {
-        // FIXME(SimonSapin): Servo uses this to implement CSS multicol / fragmentation
-        // Maybe this isn’t useful for Gecko?
-    }
 }
 
 /// A wrapper on top of two kind of iterators, depending on the parent being

components/style/matching.rs

@@ -504,7 +504,6 @@ pub trait MatchMethods : TElement {
         mut new_styles: ResolvedElementStyles,
         important_rules_changed: bool,
     ) -> ChildCascadeRequirement {
-        use dom::TNode;
         use std::cmp;
 
         self.process_animations(
@@ -518,24 +517,6 @@ pub trait MatchMethods : TElement {
         // First of all, update the styles.
         let old_styles = data.set_styles(new_styles);
 
-        // Propagate the "can be fragmented" bit. It would be nice to
-        // encapsulate this better.
-        if cfg!(feature = "servo") {
-            let layout_parent =
-                self.inheritance_parent().map(|e| e.layout_parent());
-            let layout_parent_data =
-                layout_parent.as_ref().and_then(|e| e.borrow_data());
-            let layout_parent_style =
-                layout_parent_data.as_ref().map(|d| d.styles.primary());
-
-            if let Some(ref p) = layout_parent_style {
-                let can_be_fragmented =
-                    p.is_multicol() ||
-                    layout_parent.as_ref().unwrap().as_node().can_be_fragmented();
-                unsafe { self.as_node().set_can_be_fragmented(can_be_fragmented); }
-            }
-        }
-
         let new_primary_style = data.styles.primary.as_ref().unwrap();
 
         let mut cascade_requirement = ChildCascadeRequirement::CanSkipCascade;

components/style/properties/computed_value_flags.rs

@@ -62,6 +62,11 @@ bitflags! {
 
         /// A flag to mark a style which is a visited style.
         const IS_STYLE_IF_VISITED = 1 << 9;
+
+        /// Whether the style or any of the ancestors has a multicol style.
+        ///
+        /// Only used in Servo.
+        const CAN_BE_FRAGMENTED = 1 << 10;
     }
 }
 

components/style/properties/gecko.mako.rs

@@ -300,10 +300,6 @@ impl ComputedValuesInner {
         !self.get_box().gecko.mBinding.mRawPtr.is_null()
     }
 
-    // FIXME(bholley): Implement this properly.
-    #[inline]
-    pub fn is_multicol(&self) -> bool { false }
-
     pub fn to_declaration_block(&self, property: PropertyDeclarationId) -> PropertyDeclarationBlock {
         let value = match property {
             % for prop in data.longhands:

components/style/properties/properties.mako.rs

@@ -2049,6 +2049,18 @@ pub mod style_structs {
                     .take(self.transition_property_count())
                     .any(|t| t.seconds() > 0.)
             }
+        % elif style_struct.name == "Column":
+            /// Whether this is a multicol style.
+            #[cfg(feature = "servo")]
+            pub fn is_multicol(&self) -> bool {
+                match self.column_width {
+                    Either::First(_width) => true,
+                    Either::Second(_auto) => match self.column_count {
+                        Either::First(_n) => true,
+                        Either::Second(_auto) => false,
+                    }
+                }
+            }
         % endif
     }
 
@@ -2267,17 +2279,16 @@ impl ComputedValuesInner {
         }
     }
 
+    /// Whether the current style or any of its ancestors is multicolumn.
+    #[inline]
+    pub fn can_be_fragmented(&self) -> bool {
+        self.flags.contains(ComputedValueFlags::CAN_BE_FRAGMENTED)
+    }
+
     /// Whether the current style is multicolumn.
     #[inline]
     pub fn is_multicol(&self) -> bool {
-        let style = self.get_column();
-        match style.column_width {
-            Either::First(_width) => true,
-            Either::Second(_auto) => match style.column_count {
-                Either::First(_n) => true,
-                Either::Second(_auto) => false,
-            }
-        }
+        self.get_column().is_multicol()
     }
 
     /// Resolves the currentColor keyword.

components/style/style_adjuster.rs

@@ -104,6 +104,15 @@ impl<'a, 'b: 'a> StyleAdjuster<'a, 'b> {
             self.style.is_pseudo_element() {
             self.style.flags.insert(ComputedValueFlags::IS_IN_PSEUDO_ELEMENT_SUBTREE);
         }
+
+        #[cfg(feature = "servo")]
+        {
+            if self.style.inherited_flags().contains(ComputedValueFlags::CAN_BE_FRAGMENTED) ||
+                self.style.get_parent_column().is_multicol()
+            {
+                self.style.flags.insert(ComputedValueFlags::CAN_BE_FRAGMENTED);
+            }
+        }
     }
 
     /// Adjust the style for text style.