diff --git a/src/components/main/layout/float.rs b/src/components/main/layout/float.rs
index df97ebf9112..8aee9e4837a 100644
--- a/src/components/main/layout/float.rs
+++ b/src/components/main/layout/float.rs
@@ -302,6 +302,7 @@ impl FlowContext for FloatFlow {
 
             position.size.height = height;
         }
+
     }
 
     fn collapse_margins(&mut self,
diff --git a/src/components/main/layout/flow.rs b/src/components/main/layout/flow.rs
index 472950b4a96..35e06902f8f 100644
--- a/src/components/main/layout/flow.rs
+++ b/src/components/main/layout/flow.rs
@@ -90,12 +90,12 @@ pub trait FlowContext {
         fail!("assign_widths not yet implemented")
     }
 
-    /// Pass 3 of reflow: computes height.
+    /// Pass 3a of reflow: computes height.
     fn assign_height(&mut self, _ctx: &mut LayoutContext) {
         fail!("assign_height not yet implemented")
     }
 
-    /// In-order version of pass 3 of reflow: computes heights with floats present.
+    /// In-order version of pass 3a of reflow: computes heights with floats present.
     fn assign_height_inorder(&mut self, _ctx: &mut LayoutContext) {
         fail!("assign_height_inorder not yet implemented")
     }
@@ -190,6 +190,9 @@ pub trait MutableFlowUtils {
     /// Removes the last child of this flow and destroys it.
     fn remove_last(self);
 
+    /// Computes the overflow region for this flow.
+    fn store_overflow(self, _: &mut LayoutContext);
+
     /// Builds a display list for this flow and its children.
     fn build_display_list<E:ExtraDisplayListData>(
                           self,
@@ -315,7 +318,15 @@ pub struct FlowData {
     // layout; maybe combine into a single enum to save space.
     min_width: Au,
     pref_width: Au,
+
+    /// The position of the upper left corner of the border box of this flow, relative to the
+    /// containing block.
     position: Rect<Au>,
+
+    /// The amount of overflow of this flow, relative to the containing block. Must include all the
+    /// pixels of all the display list items for correct invalidation.
+    overflow: Rect<Au>,
+
     floats_in: FloatContext,
     floats_out: FloatContext,
     num_floats: uint,
@@ -353,6 +364,7 @@ impl FlowData {
             min_width: Au::new(0),
             pref_width: Au::new(0),
             position: Au::zero_rect(),
+            overflow: Au::zero_rect(),
             floats_in: Invalid,
             floats_out: Invalid,
             num_floats: 0,
@@ -467,6 +479,17 @@ impl<'self> MutableFlowUtils for &'self mut FlowContext {
         let _ = mut_base(self).children.pop_back();
     }
 
+    fn store_overflow(self, _: &mut LayoutContext) {
+        let my_position = mut_base(self).position;
+        let mut overflow = my_position;
+        for kid in mut_base(self).child_iter() {
+            let mut kid_overflow = base(*kid).overflow;
+            kid_overflow = kid_overflow.translate(&my_position.origin);
+            overflow = overflow.union(&kid_overflow)
+        }
+        mut_base(self).overflow = overflow
+    }
+
     fn build_display_list<E:ExtraDisplayListData>(
                           self,
                           builder: &DisplayListBuilder,
diff --git a/src/components/main/layout/layout_task.rs b/src/components/main/layout/layout_task.rs
index cb029942710..046f2518ffe 100644
--- a/src/components/main/layout/layout_task.rs
+++ b/src/components/main/layout/layout_task.rs
@@ -138,15 +138,16 @@ impl<'self> PreorderFlowTraversal for AssignWidthsTraversal<'self> {
     }
 }
 
-/// The assign-heights traversal, the last (and most expensive) part of layout computation.
-/// Determines the final heights for all layout objects. In Gecko this corresponds to
-/// `FinishAndStoreOverflow`.
-struct AssignHeightsTraversal<'self>(&'self mut LayoutContext);
+/// The assign-heights-and-store-overflow traversal, the last (and most expensive) part of layout
+/// computation. Determines the final heights for all layout objects, computes positions, and
+/// computes overflow regions. In Gecko this corresponds to `FinishAndStoreOverflow`.
+struct AssignHeightsAndStoreOverflowTraversal<'self>(&'self mut LayoutContext);
 
-impl<'self> PostorderFlowTraversal for AssignHeightsTraversal<'self> {
+impl<'self> PostorderFlowTraversal for AssignHeightsAndStoreOverflowTraversal<'self> {
     #[inline]
     fn process(&mut self, flow: &mut FlowContext) -> bool {
         flow.assign_height(**self);
+        flow.store_overflow(**self);
         true
     }
 
@@ -296,6 +297,29 @@ impl LayoutTask {
         self.stylist.add_stylesheet(sheet, AuthorOrigin);
     }
 
+    /// Performs layout constraint solving.
+    ///
+    /// This corresponds to `Reflow()` in Gecko and `layout()` in WebKit/Blink and should be
+    /// benchmarked against those two. It is marked `#[inline(never)]` to aid profiling.
+    #[inline(never)]
+    fn solve_constraints(&mut self,
+                         layout_root: &mut FlowContext,
+                         layout_context: &mut LayoutContext) {
+        let _ = layout_root.traverse_postorder(&mut BubbleWidthsTraversal(layout_context));
+
+        // FIXME(kmc): We want to do
+        //     for flow in layout_root.traverse_preorder_prune(|f|
+        //          f.restyle_damage().lacks(Reflow)) 
+        // but FloatContext values can't be reused, so we need to recompute them every time.
+        // NOTE: this currently computes borders, so any pruning should separate that operation out.
+        let _ = layout_root.traverse_preorder(&mut AssignWidthsTraversal(layout_context));
+
+        // For now, this is an inorder traversal
+        // FIXME: prune this traversal as well
+        let _ = layout_root.traverse_postorder(&mut
+            AssignHeightsAndStoreOverflowTraversal(layout_context));
+    }
+
     /// The high-level routine that performs layout tasks.
     fn handle_reflow(&mut self, data: &Reflow) {
         // FIXME: Isolate this transmutation into a "bridge" module.
@@ -365,17 +389,7 @@ impl LayoutTask {
         // Perform the primary layout passes over the flow tree to compute the locations of all
         // the boxes.
         do profile(time::LayoutMainCategory, self.profiler_chan.clone()) {
-            let _ = layout_root.traverse_postorder(&mut BubbleWidthsTraversal(&mut layout_ctx));
-
-            // FIXME(kmc): We want to do
-            //     for flow in layout_root.traverse_preorder_prune(|f| f.restyle_damage().lacks(Reflow)) 
-            // but FloatContext values can't be reused, so we need to recompute them every time.
-            // NOTE: this currently computes borders, so any pruning should separate that operation out.
-            let _ = layout_root.traverse_preorder(&mut AssignWidthsTraversal(&mut layout_ctx));
-
-            // For now, this is an inorder traversal
-            // FIXME: prune this traversal as well
-            let _ = layout_root.traverse_postorder(&mut AssignHeightsTraversal(&mut layout_ctx));
+            self.solve_constraints(layout_root, &mut layout_ctx)
         }
 
         // Build the display list if necessary, and send it to the renderer.
diff --git a/src/components/util/geometry.rs b/src/components/util/geometry.rs
index d67fae0c003..febb33660f4 100644
--- a/src/components/util/geometry.rs
+++ b/src/components/util/geometry.rs
@@ -8,9 +8,16 @@ use geom::size::Size2D;
 
 use std::num::{NumCast, One, Zero};
 
-#[deriving(Clone)]
 pub struct Au(i32);
 
+// We don't use #[deriving] here for inlining.
+impl Clone for Au {
+    #[inline]
+    fn clone(&self) -> Au {
+        Au(**self)
+    }
+}
+
 impl Eq for Au {
     #[inline]
     fn eq(&self, other: &Au) -> bool {
diff --git a/src/support/geom/rust-geom b/src/support/geom/rust-geom
index 82d56dfc632..6e4db1ea746 160000
--- a/src/support/geom/rust-geom
+++ b/src/support/geom/rust-geom
@@ -1 +1 @@
-Subproject commit 82d56dfc632fe458d126d049fd5abe856170f48d
+Subproject commit 6e4db1ea746f38fe8156bc285c89ad5c12b124a9