Position fixed displays out of normal flow.

src/components/main/layout/block.rs

@@ -55,6 +55,7 @@ pub struct BlockFlow {
     /// The associated box.
     box_: Option<Box>,
 
+    //TODO: is_fixed and is_root should be bit fields to conserve memory.
     /// Whether this block flow is the root flow.
     is_root: bool,
 
@@ -360,11 +361,26 @@ impl BlockFlow {
             margin.top = margin_top;
             margin.bottom = margin_bottom;
 
-            position.origin.y = clearance + margin.top;
-
             noncontent_height = box_.padding.get().top + box_.padding.get().bottom +
                 box_.border.get().top + box_.border.get().bottom;
-            position.size.height = height + noncontent_height;
+
+            let (y, h) = box_.get_y_coord_and_new_height_if_fixed(ctx.screen_size.size.height,
+                                                                 height, clearance + margin.top, self.is_fixed);
+            position.origin.y = y;
+            height = h;
+
+            if self.is_fixed { 
+                for kid in self.base.child_iter() {
+                    let child_node = flow::mut_base(*kid);
+                    child_node.position.origin.y = position.origin.y + top_offset;
+                }
+            }
+
+            position.size.height = if self.is_fixed {
+                height
+            } else {
+                height + noncontent_height
+            };
 
             noncontent_height = noncontent_height + clearance + margin.top + margin.bottom;
 
@@ -372,7 +388,11 @@ impl BlockFlow {
             box_.margin.set(margin);
         }
 
-        self.base.position.size.height = height + noncontent_height;
+        self.base.position.size.height = if self.is_fixed {
+            height
+        } else {
+            height + noncontent_height
+        };
 
         if inorder {
             let extra_height = height - (cur_y - top_offset) + bottom_offset;
@@ -489,9 +509,9 @@ impl BlockFlow {
         for box_ in self.box_.iter() {
             box_.build_display_list(builder, dirty, self.base.abs_position, (&*self) as &Flow, list)
         }
-
         // TODO: handle any out-of-flow elements
         let this_position = self.base.abs_position;
+
         for child in self.base.child_iter() {
             let child_base = flow::mut_base(*child);
             child_base.abs_position = this_position + child_base.position.origin;
@@ -600,7 +620,7 @@ impl Flow for BlockFlow {
                },
                self.base.id);
 
-        if self.is_root || self.is_fixed {
+        if self.is_root {
             debug!("Setting root position");
             self.base.position.origin = Au::zero_point();
             self.base.position.size.width = ctx.screen_size.size.width;
@@ -648,12 +668,19 @@ impl Flow for BlockFlow {
                                               margin_bottom,
                                               margin_left));
 
-            x_offset = box_.offset();
-            remaining_width = width;
+            let (x, w) = box_.get_x_coord_and_new_width_if_fixed(ctx.screen_size.size.width, 
+                                                            ctx.screen_size.size.height, width, box_.offset(), self.is_fixed);
+            x_offset = x;
+            remaining_width = w;
 
             // The associated box is the border box of this flow.
             let mut position_ref = box_.position.borrow_mut();
-            position_ref.get().origin.x = box_.margin.get().left;
+            if self.is_fixed {
+                position_ref.get().origin.x = x_offset + box_.margin.get().left;
+                x_offset = x_offset + box_.padding.get().left;
+            } else {
+                position_ref.get().origin.x = box_.margin.get().left;
+            }
             let padding_and_borders = box_.padding.get().left + box_.padding.get().right +
                 box_.border.get().left + box_.border.get().right;
             position_ref.get().size.width = remaining_width + padding_and_borders;

src/components/main/layout/box_.rs

@@ -37,7 +37,7 @@ use layout::display_list_builder::{DisplayListBuilder, ExtraDisplayListData, ToG
 use layout::float_context::{ClearType, ClearLeft, ClearRight, ClearBoth};
 use layout::flow::Flow;
 use layout::flow;
-use layout::model::{MaybeAuto, specified};
+use layout::model::{MaybeAuto, Auto, specified};
 use layout::util::OpaqueNode;
 use layout::wrapper::LayoutNode;
 
@@ -306,6 +306,71 @@ impl Box {
         }
     }
 
+    //TODO(ibnc) take into account padding.
+    pub fn get_y_coord_and_new_height_if_fixed(&self, 
+                                               screen_height: Au,
+                                               mut height: Au,
+                                               mut y: Au,
+                                               is_fixed: bool)
+                                               -> (Au, Au) {
+        if is_fixed { 
+            let position_offsets = self.position_offsets.get();
+            match (position_offsets.top, position_offsets.bottom) {
+                (Au(0), Au(0)) => {}
+                (Au(0), _) => {
+                    y = screen_height - position_offsets.bottom - height;
+                }
+                (_, Au(0)) => {
+                    y = position_offsets.top;
+                }
+                (_, _) => {
+                    y = position_offsets.top;
+                    match MaybeAuto::from_style(self.style().Box.height, Au(0)) {
+                        Auto => {
+                            height = screen_height - position_offsets.top - position_offsets.bottom;
+                        }
+                        _ => {}
+                    }
+                }
+            }
+        }
+        return (y, height);
+    }
+
+    //TODO(ibnc) removing padding when width needs to be stretched.
+    pub fn get_x_coord_and_new_width_if_fixed(&self,
+                                              screen_width: Au,
+                                              screen_height: Au,
+                                              mut width: Au,
+                                              mut x: Au,
+                                              is_fixed: bool)
+                                              -> (Au, Au) {
+        if is_fixed {
+            self.compute_positioned_offsets(self.style(), screen_width, screen_height);
+            let position_offsets = self.position_offsets.get();
+
+            match (position_offsets.left, position_offsets.right) {
+                (Au(0), Au(0)) => {}
+                (_, Au(0)) => {
+                   x = position_offsets.left;
+                }
+                (Au(0), _) => {
+                    x = screen_width - position_offsets.right - width;
+                }
+                (_, _) => {
+                    x = position_offsets.left;
+                    match MaybeAuto::from_style(self.style().Box.width, Au(0)) {
+                        Auto => {
+                            width = screen_width - position_offsets.left - position_offsets.right;
+                        }
+                        _ => {}
+                    }
+                }
+            }
+        }
+        return (x, width);
+    }
+
     /// Transforms this box into another box of the given type, with the given size, preserving all
     /// the other data.
     pub fn transform(&self, size: Size2D<Au>, specific: SpecificBoxInfo) -> Box {
@@ -375,12 +440,16 @@ impl Box {
                                                  style.Border.border_left_style)))
     }
 
-    pub fn compute_positioned_offset(&self, style: &ComputedValues) {
+    pub fn compute_positioned_offsets(&self, style: &ComputedValues, containing_width: Au, containing_height: Au) {
         self.position_offsets.set(SideOffsets2D::new(
-                MaybeAuto::from_style(style.PositionOffsets.top, Au::new(0)).specified_or_zero(),
-                MaybeAuto::from_style(style.PositionOffsets.right, Au::new(0)).specified_or_zero(),
-                MaybeAuto::from_style(style.PositionOffsets.bottom, Au::new(0)).specified_or_zero(),
-                MaybeAuto::from_style(style.PositionOffsets.left, Au::new(0)).specified_or_zero()));
+                MaybeAuto::from_style(style.PositionOffsets.top, containing_height)
+                .specified_or_zero(),
+                MaybeAuto::from_style(style.PositionOffsets.right, containing_width)
+                .specified_or_zero(),
+                MaybeAuto::from_style(style.PositionOffsets.bottom, containing_height)
+                .specified_or_zero(),
+                MaybeAuto::from_style(style.PositionOffsets.left, containing_width)
+                .specified_or_zero()));
     }
 
     /// Populates the box model padding parameters from the given computed style.

src/components/main/layout/construct.rs

@@ -344,10 +344,10 @@ impl<'fc> FlowConstructor<'fc> {
     /// Builds a flow for a node with `display: block`. This yields a `BlockFlow` with possibly
     /// other `BlockFlow`s or `InlineFlow`s underneath it, depending on whether {ib} splits needed
     /// to happen.
-    fn build_flow_for_block(&mut self, node: LayoutNode, is_fixed: bool) -> ~Flow: {
+    fn build_flow_for_block(&mut self, node: LayoutNode, is_fixed: bool) -> ~Flow {
         let base = BaseFlow::new(self.next_flow_id(), node);
         let box_ = self.build_box_for_node(node);
-        let mut flow = ~BlockFlow::from_box(base, box_, is_fixed) as ~Flow:;
+        let mut flow = ~BlockFlow::from_box(base, box_, is_fixed) as ~Flow;
         self.build_children_of_block_flow(&mut flow, node);
         flow
     }

src/components/main/layout/layout_task.rs

@@ -378,7 +378,7 @@ impl LayoutTask {
         };
         flow.mark_as_root();
         flow
-                           }
+    }
 
     /// Performs layout constraint solving.
     ///
@@ -399,7 +399,7 @@ impl LayoutTask {
         // For now, this is an inorder traversal
         // FIXME: prune this traversal as well
         let _ = layout_root.traverse_postorder(&mut
-                                               AssignHeightsAndStoreOverflowTraversal(layout_context));
+            AssignHeightsAndStoreOverflowTraversal(layout_context));
     }
 
     /// The high-level routine that performs layout tasks.

src/test/ref/basic.list

@@ -21,3 +21,4 @@
 == inline_text_align_a.html inline_text_align_b.html
 == font_size_em.html font_size_em_ref.html
 == font_size_percentage.html font_size_em_ref.html
+== position_fixed_a.html position_fixed_b.html

src/test/ref/position_fixed_a.html

@@ -0,0 +1,43 @@
+<html>
+  <head>
+    <style>
+    .container {
+      display: block;
+      background: blue;
+    }
+    .fixed_block {
+      background: green;
+      position: fixed;
+    }
+    .positioned_fixed_block {
+      background: yellow;
+      position: fixed;
+      top: 50%;
+      right: 25px;
+    }
+    .sized_fixed_block {
+      background: red;
+      position: fixed;
+      height: 50px;
+      width: 50px;
+      left: 5px;
+      right: 10px;
+      top: 100px;
+      bottom: 30px;
+    }
+    .stretched_fixed_block {
+      position: fixed;
+      background: black;
+      top: 100px;
+      bottom: 30px;
+    }
+  </style>
+  </head>
+  <body>
+    <div class="container">
+      <div class="fixed_block"> fixed block </div>
+      <div class="positioned_fixed_block"> positioned fixed block </div>
+      <div class="sized_fixed_block"> sized fixed block </div>
+    </div>
+  </body>
+</html>

src/test/ref/position_fixed_b.html

@@ -0,0 +1,43 @@
+<html>
+  <head>
+    <style>
+    .container {
+      display: block;
+      background: blue;
+    }
+    .fixed_block {
+      background: green;
+      position: fixed;
+    }
+    .positioned_fixed_block {
+      background: yellow;
+      position: fixed;
+      top: 50%;
+      right: 25px;
+    }
+    .sized_fixed_block {
+      background: red;
+      position: fixed;
+      height: 50px;
+      width: 50px;
+      left: 5px;
+      right: 10px;
+      top: 100px;
+      bottom: 30px;
+    }
+    .stretched_fixed_block {
+      position: fixed;
+      background: black;
+      top: 100px;
+      bottom: 30px;
+    }
+  </style>
+  </head>
+  <body>
+    <div class="container">
+      <div class="fixed_block"> fixed block </div>
+      <div class="positioned_fixed_block"> positioned fixed block </div>
+      <div class="sized_fixed_block"> sized fixed block </div>
+    </div>
+  </body>
+</html>