script: Keep the DOM-side viewport up to date when scrolling happens in

WebRender.

This happens asynchronously, just as it does in non-WebRender mode.

This functionality is a prerequisite for doing proper display-list-based
hit testing in WebRender, since it moves the scroll offsets into Servo
(and, specifically, into the script thread, enabling iframe event
forwarding) instead of keeping them private to WebRender.

Requires servo/webrender_traits#55 and servo/webrender#277.

Partially addresses #11108.

components/gfx_traits/lib.rs

@@ -149,3 +149,58 @@ impl FrameTreeId {
         self.0 += 1;
     }
 }
+
+/// A unique ID for every stacking context.
+#[derive(Clone, Copy, Debug, Deserialize, Eq, Hash, HeapSizeOf, PartialEq, Serialize)]
+pub struct StackingContextId(
+    /// The identifier for this StackingContext, derived from the Flow's memory address
+    /// and fragment type.  As a space optimization, these are combined into a single word.
+    usize
+);
+
+impl StackingContextId {
+    #[inline(always)]
+    pub fn new(id: usize) -> StackingContextId {
+        StackingContextId::new_of_type(id, FragmentType::FragmentBody)
+    }
+
+    #[inline(always)]
+    pub fn new_of_type(id: usize, fragment_type: FragmentType) -> StackingContextId {
+        debug_assert_eq!(id & fragment_type as usize, 0);
+        StackingContextId(id | fragment_type as usize)
+    }
+
+    #[inline]
+    pub fn fragment_type(&self) -> FragmentType {
+        FragmentType::from_usize(self.0 & 3)
+    }
+
+    #[inline]
+    pub fn id(&self) -> usize {
+        self.0 & !3
+    }
+}
+
+
+#[derive(Clone, Debug, PartialEq, Eq, Copy, Hash, Deserialize, Serialize, HeapSizeOf)]
+pub enum FragmentType {
+    /// A StackingContext for the fragment body itself.
+    FragmentBody,
+    /// A StackingContext created to contain ::before pseudo-element content.
+    BeforePseudoContent,
+    /// A StackingContext created to contain ::after pseudo-element content.
+    AfterPseudoContent,
+}
+
+impl FragmentType {
+    #[inline]
+    pub fn from_usize(n: usize) -> FragmentType {
+        debug_assert!(n < 3);
+        match n {
+            0 => FragmentType::FragmentBody,
+            1 => FragmentType::BeforePseudoContent,
+            _ => FragmentType::AfterPseudoContent,
+        }
+    }
+}
+