Bug 1227766 patch 3 - Make will-change cause creation of a containing block for fixed and absolutely positioned elements when needed. r=dholbert

--HG--
extra : commitid : 8VbTSvfxslI

layout/style/nsStyleStruct.cpp

@@ -3001,14 +3001,19 @@ nsChangeHint nsStyleDisplay::CalcDifference(const nsStyleDisplay& aOther) const
   // test above handles relevant changes in the
   // NS_STYLE_WILL_CHANGE_TRANSFORM bit, which in turn handles frame 
   // reconstruction for changes in the containing block of 
-  // fixed-positioned elements.  Other than that, all changes to 
-  // 'will-change' can be handled by a repaint.
+  // fixed-positioned elements.
   uint8_t willChangeBitsChanged =
     mWillChangeBitField ^ aOther.mWillChangeBitField;
-  if (willChangeBitsChanged) {
+  if (willChangeBitsChanged & (NS_STYLE_WILL_CHANGE_STACKING_CONTEXT |
+                               NS_STYLE_WILL_CHANGE_SCROLL |
+                               NS_STYLE_WILL_CHANGE_OPACITY)) {
     NS_UpdateHint(hint, nsChangeHint_RepaintFrame);
   }
 
+  if (willChangeBitsChanged & NS_STYLE_WILL_CHANGE_FIXPOS_CB) {
+    NS_UpdateHint(hint, nsChangeHint_UpdateContainingBlock);
+  }
+
   // Note:  Our current behavior for handling changes to the
   // transition-duration, transition-delay, and transition-timing-function
   // properties is to do nothing.  In other words, the transition

layout/style/nsStyleStructInlines.h

@@ -144,6 +144,7 @@ nsStyleDisplay::IsFixedPosContainingBlock(const nsIFrame* aContextFrame) const
                "unexpected aContextFrame");
   return (IsContainPaint() || HasTransform(aContextFrame) ||
           HasPerspectiveStyle() ||
+          (mWillChangeBitField & NS_STYLE_WILL_CHANGE_FIXPOS_CB) ||
           aContextFrame->StyleSVGReset()->HasFilters()) &&
       !aContextFrame->IsSVGText();
 }