--- a/layout/base/nsPresContext.h
+++ b/layout/base/nsPresContext.h
@@ -450,17 +450,17 @@ public:
   virtual void Detach();
 
   /**
    * Get the visible area associated with this presentation context.
    * This is the size of the visible area that is used for
    * presenting the document. The returned value is in the standard
    * nscoord units (as scaled by the device context).
    */
-  nsRect GetVisibleArea() { return mVisibleArea; }
+  nsRect GetVisibleArea() const { return mVisibleArea; }
 
   /**
    * Set the currently visible area. The units for r are standard
    * nscoord units (as scaled by the device context).
    */
   void SetVisibleArea(const nsRect& r) {
     if (!r.IsEqualEdges(mVisibleArea)) {
       mVisibleArea = r;