From b809aae74b5b5b993cca1ee8622b8800fcdd1c61 Mon Sep 17 00:00:00 2001
From: Patrick Walton <pcwalton@mimiga.net>
Date: Mon, 24 Oct 2016 17:31:55 -0700
Subject: [PATCH] layout: Add a new `MARGINS_CANNOT_COLLAPSE` flow flag.

---
 components/layout/block.rs | 6 ++++--
 components/layout/flow.rs  | 3 +++
 2 files changed, 7 insertions(+), 2 deletions(-)

diff --git a/components/layout/block.rs b/components/layout/block.rs
index df1f001076f..1658a703182 100644
--- a/components/layout/block.rs
+++ b/components/layout/block.rs
@@ -36,7 +36,7 @@ use floats::{ClearType, FloatKind, Floats, PlacementInfo};
 use flow::{self, BaseFlow, EarlyAbsolutePositionInfo, Flow, FlowClass, ForceNonfloatedFlag};
 use flow::{BLOCK_POSITION_IS_STATIC, CLEARS_LEFT, CLEARS_RIGHT};
 use flow::{CONTAINS_TEXT_OR_REPLACED_FRAGMENTS, INLINE_POSITION_IS_STATIC};
-use flow::{FragmentationContext, PreorderFlowTraversal};
+use flow::{FragmentationContext, MARGINS_CANNOT_COLLAPSE, PreorderFlowTraversal};
 use flow::{ImmutableFlowUtils, LateAbsolutePositionInfo, MutableFlowUtils, OpaqueFlow};
 use flow::IS_ABSOLUTELY_POSITIONED;
 use flow_list::FlowList;
@@ -1911,7 +1911,9 @@ impl Flow for BlockFlow {
                 self.fragment.restyle_damage.remove(REFLOW_OUT_OF_FLOW | REFLOW);
             }
             None
-        } else if self.is_root() || self.formatting_context_type() != FormattingContextType::None {
+        } else if self.is_root() ||
+                self.formatting_context_type() != FormattingContextType::None ||
+                self.base.flags.contains(MARGINS_CANNOT_COLLAPSE) {
             // Root element margins should never be collapsed according to CSS § 8.3.1.
             debug!("assign_block_size: assigning block_size for root flow {:?}",
                    flow::base(self).debug_id());
diff --git a/components/layout/flow.rs b/components/layout/flow.rs
index 0e92de265d1..0a595cdc4aa 100644
--- a/components/layout/flow.rs
+++ b/components/layout/flow.rs
@@ -665,6 +665,9 @@ bitflags! {
 
         /// Whether this flow contains any text and/or replaced fragments.
         const CONTAINS_TEXT_OR_REPLACED_FRAGMENTS = 0b0001_0000_0000_0000_0000_0000,
+
+        /// Whether margins are prohibited from collapsing with this flow.
+        const MARGINS_CANNOT_COLLAPSE = 0b0010_0000_0000_0000_0000_0000,
     }
 }