From d588bfd40b3db18b3b59f23bc95c756328cc0e77 Mon Sep 17 00:00:00 2001
From: "L. David Baron" <dbaron@dbaron.org>
Date: Mon, 7 Dec 2015 18:39:41 -0500
Subject: [PATCH] Bug 1230701 - Make will-change:position create a containing
 block for absolutely-positioned elements.  r=dholbert

--HG--
extra : commitid : 99HuPIkGhg5
---
 layout/style/nsCSSPropList.h        | 3 ++-
 layout/style/nsCSSProps.h           | 9 +++++++++
 layout/style/nsRuleNode.cpp         | 6 ++++--
 layout/style/nsStyleConsts.h        | 1 +
 layout/style/nsStyleStructInlines.h | 7 +++++--
 5 files changed, 21 insertions(+), 5 deletions(-)

diff --git a/layout/style/nsCSSPropList.h b/layout/style/nsCSSPropList.h
index 66915495cda4..6c3c3fe45043 100644
--- a/layout/style/nsCSSPropList.h
+++ b/layout/style/nsCSSPropList.h
@@ -3029,7 +3029,8 @@ CSS_PROP_DISPLAY(
     Position,
     CSS_PROPERTY_PARSE_VALUE |
         // For position: sticky/fixed
-        CSS_PROPERTY_CREATES_STACKING_CONTEXT,
+        CSS_PROPERTY_CREATES_STACKING_CONTEXT |
+        CSS_PROPERTY_ABSPOS_CB,
     "",
     VARIANT_HK,
     kPositionKTable,
diff --git a/layout/style/nsCSSProps.h b/layout/style/nsCSSProps.h
index 0ee81f148387..45be06c891cc 100644
--- a/layout/style/nsCSSProps.h
+++ b/layout/style/nsCSSProps.h
@@ -262,6 +262,15 @@ static_assert((CSS_PROPERTY_PARSE_PROPERTY_MASK &
 // nsStyleDisplay::IsFixedPosContainingBlock.
 #define CSS_PROPERTY_FIXPOS_CB                    (1<<29)
 
+// This property has values that can establish a containing block for
+// absolutely positioned elements.
+// This should be set for any properties that can cause an element to be
+// such a containing block, as implemented in
+// nsStyleDisplay::IsAbsPosContainingBlock.
+// It does not need to be set for properties that also have
+// CSS_PROPERTY_FIXPOS_CB set.
+#define CSS_PROPERTY_ABSPOS_CB                    (1<<30)
+
 /**
  * Types of animatable values.
  */
diff --git a/layout/style/nsRuleNode.cpp b/layout/style/nsRuleNode.cpp
index 9692407e44c5..a6c0869607dc 100644
--- a/layout/style/nsRuleNode.cpp
+++ b/layout/style/nsRuleNode.cpp
@@ -6084,8 +6084,10 @@ nsRuleNode::ComputeDisplayData(void* aStartStruct,
               NS_STYLE_WILL_CHANGE_STACKING_CONTEXT;
           }
           if (nsCSSProps::PropHasFlags(prop, CSS_PROPERTY_FIXPOS_CB)) {
-            display->mWillChangeBitField |=
-              NS_STYLE_WILL_CHANGE_FIXPOS_CB;
+            display->mWillChangeBitField |= NS_STYLE_WILL_CHANGE_FIXPOS_CB;
+          }
+          if (nsCSSProps::PropHasFlags(prop, CSS_PROPERTY_ABSPOS_CB)) {
+            display->mWillChangeBitField |= NS_STYLE_WILL_CHANGE_ABSPOS_CB;
           }
         }
       }
diff --git a/layout/style/nsStyleConsts.h b/layout/style/nsStyleConsts.h
index e71fe7029593..2b0874e74225 100644
--- a/layout/style/nsStyleConsts.h
+++ b/layout/style/nsStyleConsts.h
@@ -244,6 +244,7 @@ enum class StyleBoxSizing : uint8_t {
 #define NS_STYLE_WILL_CHANGE_SCROLL             (1<<2)
 #define NS_STYLE_WILL_CHANGE_OPACITY            (1<<3)
 #define NS_STYLE_WILL_CHANGE_FIXPOS_CB          (1<<4)
+#define NS_STYLE_WILL_CHANGE_ABSPOS_CB          (1<<5)
 
 // See AnimationEffectReadOnly.webidl
 // and mozilla/dom/AnimationEffectReadOnlyBinding.h
diff --git a/layout/style/nsStyleStructInlines.h b/layout/style/nsStyleStructInlines.h
index 963856e2eaad..6e845646ad66 100644
--- a/layout/style/nsStyleStructInlines.h
+++ b/layout/style/nsStyleStructInlines.h
@@ -154,8 +154,11 @@ nsStyleDisplay::IsAbsPosContainingBlock(const nsIFrame* aContextFrame) const
 {
   NS_ASSERTION(aContextFrame->StyleDisplay() == this,
                "unexpected aContextFrame");
-  return ((IsAbsolutelyPositionedStyle() || IsRelativelyPositionedStyle()) &&
-          !aContextFrame->IsSVGText()) || IsFixedPosContainingBlock(aContextFrame);
+  return ((IsAbsolutelyPositionedStyle() ||
+           IsRelativelyPositionedStyle() ||
+           (mWillChangeBitField & NS_STYLE_WILL_CHANGE_ABSPOS_CB)) &&
+          !aContextFrame->IsSVGText()) ||
+         IsFixedPosContainingBlock(aContextFrame);
 }
 
 bool