auto merge of #801 : eschweic/servo/comp-fixed-pos, r=metajack

Some changes that need to happen for #782.

src/components/main/compositing/compositor_layer.rs

@@ -47,6 +47,8 @@ pub struct CompositorLayer {
     /// A monotonically increasing counter that keeps track of the current epoch.
     /// add_buffer() calls that don't match the current epoch will be ignored.
     epoch: Epoch,
+    /// The behavior of this layer when a scroll message is received. 
+    scroll_behavior: ScrollBehavior,
 }
 
 /// Helper struct for keeping CompositorLayer children organized.
@@ -65,6 +67,16 @@ enum MaybeQuadtree {
     NoTree(uint, Option<uint>),
 }
 
+/// Determines the behavior of the layer when a scroll message is recieved.
+enum ScrollBehavior {
+    /// Normal scrolling behavior.
+    Scroll,
+    /// Scrolling messages targeted at this layer are ignored, but can be
+    /// passed on to child layers.
+    FixedPosition,
+}
+               
+
 impl CompositorLayer {
     /// Creates a new CompositorLayer with an optional page size. If no page size is given,
     /// the layer is initially hidden and initialized without a quadtree.
@@ -85,6 +97,7 @@ impl CompositorLayer {
             root_layer: @mut ContainerLayer(),
             hidden: true,
             epoch: Epoch(0),
+            scroll_behavior: Scroll,
         }
     }
     
@@ -141,6 +154,9 @@ impl CompositorLayer {
             }
         }
 
+        // This scroll event is mine!
+        match self.scroll_behavior {
+            Scroll => {
                 // Scroll this layer!
                 let old_origin = self.scroll_offset;
                 self.scroll_offset = self.scroll_offset + delta;
@@ -165,6 +181,9 @@ impl CompositorLayer {
                                                                           0.0));
                 true
             }
+            FixedPosition => false, // Ignore this scroll event.
+        }
+    }
 
     // Takes in a MouseWindowEvent, determines if it should be passed to children, and 
     // sends the event off to the appropriate pipeline. NB: the cursor position is in