--- a/layout/generic/nsFlexContainerFrame.cpp
+++ b/layout/generic/nsFlexContainerFrame.cpp
@@ -82,16 +82,18 @@ kAxisOrientationToSidesMap[eNumAxisOrien
 // Returns true iff the given nsStyleDisplay has display:-webkit-{inline-}-box.
 static inline bool
 IsDisplayValueLegacyBox(const nsStyleDisplay* aStyleDisp)
 {
   return aStyleDisp->mDisplay == mozilla::StyleDisplay::WebkitBox ||
     aStyleDisp->mDisplay == mozilla::StyleDisplay::WebkitInlineBox;
 }
 
+// XXXdholbert This will be merged into Init(), in a later patch in this series
+// (after all callers have been converted to check frame state bit).
 /* static */ bool
 nsFlexContainerFrame::IsLegacyBox(const nsIFrame* aFrame)
 {
   nsStyleContext* styleContext = aFrame->StyleContext();
   const nsStyleDisplay* styleDisp = styleContext->StyleDisplay();
 
   // Trivial case: just check "display" directly.
   bool isLegacyBox = IsDisplayValueLegacyBox(styleDisp);
@@ -2269,16 +2271,31 @@ NS_NewFlexContainerFrame(nsIPresShell* a
 // nsFlexContainerFrame Method Implementations
 // ===========================================
 
 /* virtual */
 nsFlexContainerFrame::~nsFlexContainerFrame()
 {
 }
 
+/* virtual */
+void
+nsFlexContainerFrame::Init(nsIContent*       aContent,
+                           nsContainerFrame* aParent,
+                           nsIFrame*         aPrevInFlow)
+{
+  nsContainerFrame::Init(aContent, aParent, aPrevInFlow);
+
+  if (nsFlexContainerFrame::IsLegacyBox(this)) {
+    // Toggle frame state bit to indicate that this frame represents a
+    // legacy -webkit-{inline-}box container:
+    AddStateBits(NS_STATE_FLEX_IS_LEGACY_WEBKIT_BOX);
+  }
+}
+
 template<bool IsLessThanOrEqual(nsIFrame*, nsIFrame*)>
 /* static */ bool
 nsFlexContainerFrame::SortChildrenIfNeeded()
 {
   if (nsIFrame::IsFrameListSorted<IsLessThanOrEqual>(mFrames)) {
     return false;
   }
 

--- a/layout/generic/nsFlexContainerFrame.h
+++ b/layout/generic/nsFlexContainerFrame.h
@@ -53,16 +53,20 @@ public:
 
   // Forward-decls of helper classes
   class FlexItem;
   class FlexLine;
   class FlexboxAxisTracker;
   struct StrutInfo;
 
   // nsIFrame overrides
+  void Init(nsIContent*       aContent,
+            nsContainerFrame* aParent,
+            nsIFrame*         aPrevInFlow) override;
+
   virtual void BuildDisplayList(nsDisplayListBuilder*   aBuilder,
                                 const nsRect&           aDirtyRect,
                                 const nsDisplayListSet& aLists) override;
 
   virtual void Reflow(nsPresContext*           aPresContext,
                       ReflowOutput&     aDesiredSize,
                       const ReflowInput& aReflowInput,
                       nsReflowStatus&          aStatus) override;

--- a/layout/generic/nsFrameStateBits.h
+++ b/layout/generic/nsFrameStateBits.h
@@ -302,16 +302,20 @@ FRAME_STATE_BIT(Box, 61, NS_FRAME_MOUSE_
 // == Frame state bits that apply to flex container frames ====================
 
 FRAME_STATE_GROUP(FlexContainer, nsFlexContainerFrame)
 
 // Set for a flex container whose children have been reordered due to 'order'.
 // (Means that we have to be more thorough about checking them for sortedness.)
 FRAME_STATE_BIT(FlexContainer, 20, NS_STATE_FLEX_CHILDREN_REORDERED)
 
+// Set for a flex container that is emulating a legacy
+// 'display:-webkit-{inline-}box' container.
+FRAME_STATE_BIT(FlexContainer, 21, NS_STATE_FLEX_IS_LEGACY_WEBKIT_BOX)
+
 // == Frame state bits that apply to grid container frames ====================
 
 FRAME_STATE_GROUP(GridContainer, nsGridContainerFrame)
 
 // True iff the normal flow children are already in CSS 'order' in the
 // order they occur in the child frame list.
 FRAME_STATE_BIT(GridContainer, 20, NS_STATE_GRID_NORMAL_FLOW_CHILDREN_IN_CSS_ORDER)