Use reference frames explicitly for fixed positioning

Now that WebRender gives us reference frame ClipIds, we can use those to
implement fixed positioning in Servo. This will allow us to remove the
feature from WebRender entirely.

components/layout/display_list/items.rs

@@ -38,14 +38,25 @@ pub static BLUR_INFLATION_FACTOR: i32 = 3;
 /// An index into the vector of ClipScrollNodes. During WebRender conversion these nodes
 /// are given ClipIds.
 #[derive(Clone, Copy, Debug, PartialEq, Serialize)]
-pub struct ClipScrollNodeIndex(pub usize);
+pub struct ClipScrollNodeIndex(usize);
 
 impl ClipScrollNodeIndex {
+    pub fn root_scroll_node() -> ClipScrollNodeIndex {
+        ClipScrollNodeIndex(1)
+    }
+
+    pub fn root_reference_frame() -> ClipScrollNodeIndex {
+        ClipScrollNodeIndex(0)
+    }
+
+    pub fn new(index: usize) -> ClipScrollNodeIndex {
+        assert_ne!(index, 0, "Use the root_reference_frame constructor");
+        assert_ne!(index, 1, "Use the root_scroll_node constructor");
+        ClipScrollNodeIndex(index)
+    }
+
     pub fn is_root_scroll_node(&self) -> bool {
-        match *self {
-            ClipScrollNodeIndex(0) => true,
-            _ => false,
-        }
+        *self == Self::root_scroll_node()
     }
 
     pub fn to_define_item(&self) -> DisplayItem {
@@ -54,6 +65,10 @@ impl ClipScrollNodeIndex {
             node_index: *self,
         }))
     }
+
+    pub fn to_index(self) -> usize {
+        self.0
+    }
 }
 
 /// A set of indices into the clip scroll node vector for a given item.
@@ -193,6 +208,9 @@ pub struct StackingContext {
 
     /// The clip and scroll info for this StackingContext.
     pub parent_clipping_and_scrolling: ClippingAndScrolling,
+
+    /// The index of the reference frame that this stacking context estalishes.
+    pub established_reference_frame: Option<ClipScrollNodeIndex>,
 }
 
 impl StackingContext {
@@ -211,6 +229,7 @@ impl StackingContext {
         perspective: Option<LayoutTransform>,
         scroll_policy: ScrollPolicy,
         parent_clipping_and_scrolling: ClippingAndScrolling,
+        established_reference_frame: Option<ClipScrollNodeIndex>,
     ) -> StackingContext {
         StackingContext {
             id,
@@ -225,6 +244,7 @@ impl StackingContext {
             perspective,
             scroll_policy,
             parent_clipping_and_scrolling,
+            established_reference_frame,
         }
     }
 
@@ -242,7 +262,8 @@ impl StackingContext {
             TransformStyle::Flat,
             None,
             ScrollPolicy::Scrollable,
-            ClippingAndScrolling::simple(ClipScrollNodeIndex(0)),
+            ClippingAndScrolling::simple(ClipScrollNodeIndex::root_scroll_node()),
+            None,
         )
     }
 
@@ -309,15 +330,16 @@ impl fmt::Debug for StackingContext {
     }
 }
 
-#[derive(Clone, Debug, Serialize)]
+#[derive(Clone, Debug, PartialEq, Serialize)]
 pub struct StickyFrameData {
     pub margins: SideOffsets2D<Option<f32>>,
     pub vertical_offset_bounds: StickyOffsetBounds,
     pub horizontal_offset_bounds: StickyOffsetBounds,
 }
 
-#[derive(Clone, Debug, Serialize)]
+#[derive(Clone, Debug, PartialEq, Serialize)]
 pub enum ClipScrollNodeType {
+    Placeholder,
     ScrollFrame(ScrollSensitivity, ExternalScrollId),
     StickyFrame(StickyFrameData),
     Clip,
@@ -339,6 +361,21 @@ pub struct ClipScrollNode {
     pub node_type: ClipScrollNodeType,
 }
 
+impl ClipScrollNode {
+    pub fn placeholder() -> ClipScrollNode {
+        ClipScrollNode {
+            parent_index: ClipScrollNodeIndex(0),
+            clip: ClippingRegion::from_rect(LayoutRect::zero()),
+            content_rect: LayoutRect::zero(),
+            node_type: ClipScrollNodeType::Placeholder,
+        }
+    }
+
+    pub fn is_placeholder(&self) -> bool {
+        self.node_type == ClipScrollNodeType::Placeholder
+    }
+}
+
 /// One drawing command in the list.
 #[derive(Clone, Serialize)]
 pub enum DisplayItem {
@@ -412,7 +449,8 @@ impl BaseDisplayItem {
             clip_rect: LayoutRect::max_rect(),
             section: DisplayListSection::Content,
             stacking_context_id: StackingContextId::root(),
-            clipping_and_scrolling: ClippingAndScrolling::simple(ClipScrollNodeIndex(0)),
+            clipping_and_scrolling:
+                ClippingAndScrolling::simple(ClipScrollNodeIndex::root_scroll_node()),
         }
     }
 }