--- a/layout/generic/nsHTMLReflowState.cpp
+++ b/layout/generic/nsHTMLReflowState.cpp
@@ -1997,17 +1997,17 @@ CalcQuirkContainingBlockHeight(const nsH
   return std::max(result, 0);
 }
 
 // Called by InitConstraints() to compute the containing block rectangle for
 // the element. Handles the special logic for absolutely positioned elements
 LogicalSize
 nsHTMLReflowState::ComputeContainingBlockRectangle(
                      nsPresContext*           aPresContext,
-                     const nsHTMLReflowState* aContainingBlockRS)
+                     const nsHTMLReflowState* aContainingBlockRS) const
 {
   // Unless the element is absolutely positioned, the containing block is
   // formed by the content edge of the nearest block-level ancestor
   LogicalSize cbSize = aContainingBlockRS->ComputedSize();
 
   WritingMode wm = aContainingBlockRS->GetWritingMode();
 
   // mFrameType for abs-pos tables is NS_CSS_FRAME_TYPE_BLOCK, so we need to

--- a/layout/generic/nsHTMLReflowState.h
+++ b/layout/generic/nsHTMLReflowState.h
@@ -731,18 +731,18 @@ public:
    */
   static nscoord CalcLineHeight(nsIContent* aContent,
                                 nsStyleContext* aStyleContext,
                                 nscoord aBlockBSize,
                                 float aFontSizeInflation);
 
 
   mozilla::LogicalSize ComputeContainingBlockRectangle(
-         nsPresContext*          aPresContext,
-         const nsHTMLReflowState* aContainingBlockRS);
+         nsPresContext*           aPresContext,
+         const nsHTMLReflowState* aContainingBlockRS) const;
 
   /**
    * Apply the mComputed(Min/Max)Width constraints to the content
    * size computed so far.
    */
   nscoord ApplyMinMaxWidth(nscoord aWidth) const {
     if (NS_UNCONSTRAINEDSIZE != ComputedMaxWidth()) {
       aWidth = std::min(aWidth, ComputedMaxWidth());