servo: Merge #7563 - Layerize StackingContexts that are on top of layers (from mrobinson:layerize-stacking-contexts); r=pcwalton

StackingContexts that should be painted on top of StackingContexts that
are already layerized should automatically get their own layer. This
will ensure proper painting order.

Source-Repo: https://github.com/servo/servo
Source-Revision: c0381c732569b9abe6282c6c750533bc271a2019

servo/components/msg/compositor_msg.rs

@@ -36,19 +36,35 @@ impl FrameTreeId {
 }
 
 #[derive(Clone, PartialEq, Eq, Copy, Hash, Deserialize, Serialize, HeapSizeOf)]
-pub struct LayerId(pub usize, pub u32);
+pub struct LayerId(
+    /// A base layer ID, currently derived from DOM element pointer address.
+    pub usize,
+
+    /// FIXME(#2010, pcwalton): A marker for overflow scroll layers.
+    pub u32,
+
+    /// A sub ID, which is used for synthesizing new layers for content that
+    /// belongs on top of this layer. This prevents accidentally making colliding
+    /// layer ids.
+    pub u32
+);
 
 impl Debug for LayerId {
     fn fmt(&self, f: &mut Formatter) -> fmt::Result {
-        let LayerId(a, b) = *self;
-        write!(f, "Layer({}, {})", a, b)
+        let LayerId(a, b, c) = *self;
+        write!(f, "Layer({}, {}, {})", a, b, c)
     }
 }
 
 impl LayerId {
     /// FIXME(#2011, pcwalton): This is unfortunate. Maybe remove this in the future.
     pub fn null() -> LayerId {
-        LayerId(0, 0)
+        LayerId(0, 0, 0)
+    }
+
+    pub fn next_layer_id(&self) -> LayerId {
+        let LayerId(a, b, sub_id) = *self;
+        LayerId(a, b, sub_id + 1)
     }
 }