From 7a2bbf62576f30fd0eb73d7c1ff33fe4fd520e41 Mon Sep 17 00:00:00 2001
From: Ilyong Cho <ilyoan@gmail.com>
Date: Wed, 28 Aug 2013 15:34:14 +0900
Subject: [PATCH] Adjust top_offset when top margin of a element collapses with
 its first child

---
 src/components/main/layout/block.rs | 67 ++++++++++++++++++++---------
 1 file changed, 46 insertions(+), 21 deletions(-)

diff --git a/src/components/main/layout/block.rs b/src/components/main/layout/block.rs
index 13b2a675b2f..212441c8798 100644
--- a/src/components/main/layout/block.rs
+++ b/src/components/main/layout/block.rs
@@ -315,12 +315,22 @@ impl BlockFlowData {
 
         let mut collapsible = Au(0);
         let mut collapsing = Au(0);
-        let mut first_collapsing = true;
+        let mut margin_top = Au(0);
+        let mut margin_bottom = Au(0);
+        let mut top_margin_collapsible = false;
+        let mut bottom_margin_collapsible = false;
+        let mut first_inflow = true;
         for &box in self.box.iter() {
             do box.with_model |model| {
                 if !self.is_root && model.border.top == Au(0) && model.padding.top == Au(0) {
                     collapsible = model.margin.top;
+                    top_margin_collapsible = true;
                 }
+                if !self.is_root && model.border.bottom == Au(0) && model.padding.bottom == Au(0) {
+                    bottom_margin_collapsible = true;
+                }
+                margin_top = model.margin.top;
+                margin_bottom = model.margin.bottom;
             }
         }
         for kid in self.common.child_iter() {
@@ -328,19 +338,26 @@ impl BlockFlowData {
                 BlockFlow(ref info) => {
                     for &box in info.box.iter() {
                         do box.with_model |model| {
-                            collapsing = if first_collapsing && collapsible != Au(0) {
-                                // The top margin collapses with its first in-flow block-level child's
-                                // top margin if the parent has no top boder, no top padding.
-                                model.margin.top
-                            } else {
-                                // The bottom margin of an in-flow block-level element collapses 
-                                // with the top margin of its next in-flow block-level sibling.
-                                geometry::min(model.margin.top, collapsible)
-                            };
+                            // The top margin collapses with its first in-flow block-level child's
+                            // top margin if the parent has no top boder, no top padding.
+                            if first_inflow && top_margin_collapsible {
+                                // If top-margin of parent is less than top-margin of its first child, 
+                                // the parent box goes down until its top is aligned with the child.
+                                if margin_top < model.margin.top {
+                                    // TODO: The position of child floats should be updated and this
+                                    // would influence clearance as well. See #725
+                                    let extra_margin = model.margin.top - margin_top;
+                                    top_offset = top_offset + extra_margin;
+                                    margin_top = model.margin.top;
+                                }
+                            }
+                            // The bottom margin of an in-flow block-level element collapses 
+                            // with the top margin of its next in-flow block-level sibling.
+                            collapsing = geometry::min(model.margin.top, collapsible);
                             collapsible = model.margin.bottom;
                         }
                     }
-                    first_collapsing = false;
+                    first_inflow = false;
                 }
                 // Margins between a floated box and any other box do not collapse.
                 _ => {
@@ -354,17 +371,22 @@ impl BlockFlowData {
                 cur_y = cur_y + child_node.position.size.height;
             };
         }
-        // The bottom margin collapses with its first in-flow block-level child's bottom margin
+
+        // The bottom margin collapses with its last in-flow block-level child's bottom margin
         // if the parent has no bottom boder, no bottom padding.
-        for &box in self.box.iter() {
-            collapsing = do box.with_model |model| {
-                if !self.is_root && model.border.bottom == Au(0) && model.padding.bottom == Au(0) {
-                    collapsible
-                } else {
-                    Au(0)
-                }
-            };
-        }
+        collapsing = if bottom_margin_collapsible {
+            if margin_bottom < collapsible {
+                margin_bottom = collapsible;
+            }
+            collapsible
+        } else {
+            Au(0)
+        };
+        
+        // TODO: A box's own margins collapse if the 'min-height' property is zero, and it has neither
+        // top or bottom borders nor top or bottom padding, and it has a 'height' of either 0 or 'auto',
+        // and it does not contain a line box, and all of its in-flow children's margins (if any) collapse.
+
 
         let mut height = if self.is_root {
             Au::max(ctx.screen_size.size.height, cur_y)
@@ -383,6 +405,9 @@ impl BlockFlowData {
         self.box.map(|&box| {
             do box.with_mut_base |base| {
                 //The associated box is the border box of this flow
+                base.model.margin.top = margin_top;
+                base.model.margin.bottom = margin_bottom;
+
                 base.position.origin.y = clearance + base.model.margin.top;
 
                 noncontent_height = base.model.padding.top + base.model.padding.bottom +