servo: Merge #1808 - Send in the right container block size for relative positioning (from pradeep90:relative-position-container-block-size); r=pcwalton

This fixes #1757.

Source-Repo: https://github.com/servo/servo
Source-Revision: af616dba58cbf589b116b9e3cde4429df43e72cd

servo/src/components/main/layout/box_.rs

@@ -1279,6 +1279,7 @@ impl Box {
     }
 
 
+    /// TODO: What exactly does this function return? Why is it Au(0) for GenericBox?
     pub fn content_width(&self) -> Au {
         match self.specific {
             GenericBox | IframeBox(_) => Au(0),
@@ -1312,6 +1313,13 @@ impl Box {
         }
     }
 
+    /// Return the size of the content box.
+    pub fn content_box_size(&self) -> Size2D<Au> {
+        let border_box_size = self.border_box.get().size;
+        Size2D(border_box_size.width - self.noncontent_width(),
+               border_box_size.height - self.noncontent_height())
+    }
+
     /// Split box which includes new-line character
     pub fn split_by_new_line(&self) -> SplitBoxResult {
         match self.specific {

servo/src/components/main/layout/flow.rs

@@ -957,9 +957,9 @@ impl<'a> MutableFlowUtils for &'a mut Flow {
             // TODO(pradeep): Move this into a generated CB function and stuff in Flow.
             match block.box_ {
                 Some(ref box_) => {
-                    // FIXME: This should be the size of the content box (which is the
-                    // Containing Block formed by a BlockFlow), not the border box.
-                    container_block_size = box_.border_box.get().size;
+                    // The Containing Block formed by a Block for relatively
+                    // positioned descendants is the content box.
+                    container_block_size = box_.content_box_size();
 
                     abs_cb_position = if block.is_positioned() {
                         block.base.abs_position + block.generated_cb_position()