Disable incremental reflow for multicol and their descendants.

Fragmentation with dynamic updates is hard.

components/layout/construct.rs

@@ -1318,6 +1318,9 @@ impl<'a, 'ln, ConcreteThreadSafeLayoutNode: ThreadSafeLayoutNode<'ln>>
             return false
         }
 
+        if node.in_fragmentation_container() {
+            return false
+        }
 
         let mut style = node.style().clone();
         let mut data = node.mutate_layout_data().unwrap();

components/layout/wrapper.rs

@@ -50,7 +50,7 @@ use script::dom::htmlimageelement::LayoutHTMLImageElementHelpers;
 use script::dom::htmlinputelement::{HTMLInputElement, LayoutHTMLInputElementHelpers};
 use script::dom::htmltextareaelement::{HTMLTextAreaElement, LayoutHTMLTextAreaElementHelpers};
 use script::dom::node::{HAS_CHANGED, HAS_DIRTY_DESCENDANTS, IS_DIRTY};
-use script::dom::node::{LayoutNodeHelpers, Node, OpaqueStyleAndLayoutData};
+use script::dom::node::{IN_FRAGMENTATION_CONTAINER, LayoutNodeHelpers, Node, OpaqueStyleAndLayoutData};
 use script::dom::text::Text;
 use script::layout_interface::TrustedNodeAddress;
 use selectors::matching::DeclarationBlock;
@@ -226,6 +226,14 @@ impl<'ln> TNode<'ln> for ServoLayoutNode<'ln> {
         self.node.set_flag(HAS_DIRTY_DESCENDANTS, value)
     }
 
+    fn in_fragmentation_container(&self) -> bool {
+        unsafe { self.node.get_flag(IN_FRAGMENTATION_CONTAINER) }
+    }
+
+    unsafe fn set_in_fragmentation_container(&self, value: bool) {
+        self.node.set_flag(IN_FRAGMENTATION_CONTAINER, value)
+    }
+
     unsafe fn borrow_data_unchecked(&self) -> Option<*const PrivateStyleData> {
         self.borrow_layout_data_unchecked().map(|d| &(*d).style_data as *const PrivateStyleData)
     }
@@ -753,6 +761,8 @@ pub trait ThreadSafeLayoutNode<'ln> : Clone + Copy + Sized {
         }
     }
 
+    fn in_fragmentation_container(&self) -> bool;
+
     /// If this is a text node, generated content, or a form element, copies out
     /// its content. Otherwise, panics.
     ///
@@ -929,6 +939,10 @@ impl<'ln> ThreadSafeLayoutNode<'ln> for ServoThreadSafeLayoutNode<'ln> {
         }
     }
 
+    fn in_fragmentation_container(&self) -> bool {
+        self.node.in_fragmentation_container()
+    }
+
     fn text_content(&self) -> TextContent {
         if self.pseudo != PseudoElementType::Normal {
             let data = &self.borrow_layout_data().unwrap().style_data;

components/script/dom/node.rs

@@ -144,6 +144,9 @@ bitflags! {
         #[doc = "Specifies whether this node is focusable and whether it is supposed \
                  to be reachable with using sequential focus navigation."]
         const SEQUENTIALLY_FOCUSABLE = 0x20,
+
+        /// Whether any ancestor is a fragmentation container
+        const IN_FRAGMENTATION_CONTAINER = 0x40
     }
 }
 

components/style/dom.rs

@@ -129,6 +129,10 @@ pub trait TNode<'ln> : Sized + Copy + Clone {
         }
     }
 
+    fn in_fragmentation_container(&self) -> bool;
+
+    unsafe fn set_in_fragmentation_container(&self, value: bool);
+
     /// Borrows the PrivateStyleData without checks.
     #[inline(always)]
     unsafe fn borrow_data_unchecked(&self) -> Option<*const PrivateStyleData>;

components/style/matching.rs

@@ -700,6 +700,11 @@ pub trait MatchMethods<'ln> : TNode<'ln> {
             // This method needs to borrow the data as mutable, so make sure data_ref goes out of
             // scope first.
             self.set_restyle_damage(damage);
+
+            self.set_in_fragmentation_container(
+                parent.as_ref().map_or(false, |p| p.in_fragmentation_container()) ||
+                self.borrow_data().unwrap().style.as_ref().unwrap().is_multicol()
+            );
         }
     }
 }

ports/geckolib/wrapper.rs

@@ -182,6 +182,17 @@ impl<'ln> TNode<'ln> for GeckoNode<'ln> {
         unimplemented!()
     }
 
+    fn in_fragmentation_container(&self) -> bool {
+        // FIXME(SimonSapin): Servo uses this to implement CSS multicol / fragmentation
+        // Maybe this isn’t useful for Gecko?
+        false
+    }
+
+    unsafe fn set_in_fragmentation_container(&self, _value: bool) {
+        // FIXME(SimonSapin): Servo uses this to implement CSS multicol / fragmentation
+        // Maybe this isn’t useful for Gecko?
+    }
+
     #[inline(always)]
     unsafe fn borrow_data_unchecked(&self) -> Option<*const PrivateStyleData> {
         self.get_node_data().as_ref().map(|d| d.as_unsafe_cell().get() as *const PrivateStyleData)