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,
         }
     }
     
@@ -141,29 +154,35 @@ impl CompositorLayer {
             }
         }
 
-        // Scroll this layer!
+        // This scroll event is mine!
-        let old_origin = self.scroll_offset;
+        match self.scroll_behavior {
-        self.scroll_offset = self.scroll_offset + delta;
+            Scroll => {
-        
+                // Scroll this layer!
-        // bounds checking
+                let old_origin = self.scroll_offset;
-        let page_size = match self.page_size {
+                self.scroll_offset = self.scroll_offset + delta;
-            Some(size) => size,
-            None => fail!("CompositorLayer: tried to scroll with no page size set"),
-        };
-        let min_x = (window_size.width - page_size.width).min(&0.0);
-        self.scroll_offset.x = self.scroll_offset.x.clamp(&min_x, &0.0);
-        let min_y = (window_size.height - page_size.height).min(&0.0);
-        self.scroll_offset.y = self.scroll_offset.y.clamp(&min_y, &0.0);
-        
-        // check to see if we scrolled
-        if old_origin - self.scroll_offset == Point2D(0f32, 0f32) {
-            return false;
-        }
 
-        self.root_layer.common.set_transform(identity().translate(self.scroll_offset.x,
+                // bounds checking
-                                                                  self.scroll_offset.y,
+                let page_size = match self.page_size {
-                                                                  0.0));
+                    Some(size) => size,
-        true
+                    None => fail!("CompositorLayer: tried to scroll with no page size set"),
+                };
+                let min_x = (window_size.width - page_size.width).min(&0.0);
+                self.scroll_offset.x = self.scroll_offset.x.clamp(&min_x, &0.0);
+                let min_y = (window_size.height - page_size.height).min(&0.0);
+                self.scroll_offset.y = self.scroll_offset.y.clamp(&min_y, &0.0);
+
+                // check to see if we scrolled
+                if old_origin - self.scroll_offset == Point2D(0f32, 0f32) {
+                    return false;
+                }
+
+                self.root_layer.common.set_transform(identity().translate(self.scroll_offset.x,
+                                                                          self.scroll_offset.y,
+                                                                          0.0));
+                true
+            }
+            FixedPosition => false, // Ignore this scroll event.
+        }
     }
 
     // Takes in a MouseWindowEvent, determines if it should be passed to children, and