diff --git a/layout/base/nsLayoutUtils.cpp b/layout/base/nsLayoutUtils.cpp
index 9d687807b643..47b5c905b790 100644
--- a/layout/base/nsLayoutUtils.cpp
+++ b/layout/base/nsLayoutUtils.cpp
@@ -4686,11 +4686,8 @@ MinimumFontSizeFor(nsPresContext* aPresContext, nscoord aContainerWidth)
   if (sFontSizeInflationMinTwips != 0) {
     // REVIEW: Is this giving us app units and sizes *not* counting
     // viewport scaling?
-    nsDeviceContext *dx = aPresContext->DeviceContext();
-    nsRect clientRect;
-    dx->GetClientRect(clientRect); // FIXME: GetClientRect looks expensive
     float deviceWidthInches =
-      float(clientRect.width) / float(dx->AppUnitsPerPhysicalInch());
+      aPresContext->ScreenWidthInchesForFontInflation();
     byInch = NSToCoordRound(effectiveContainerWidth /
                             (deviceWidthInches * 1440 /
                              sFontSizeInflationMinTwips ));
diff --git a/layout/base/nsPresContext.cpp b/layout/base/nsPresContext.cpp
index bf3be06df124..86be2cbe1ac5 100644
--- a/layout/base/nsPresContext.cpp
+++ b/layout/base/nsPresContext.cpp
@@ -209,7 +209,8 @@ destroy_loads(nsIFrame* aKey, nsRefPtr<nsImageLoader>& aData, void* closure)
 
 nsPresContext::nsPresContext(nsIDocument* aDocument, nsPresContextType aType)
   : mType(aType), mDocument(aDocument), mMinFontSize(0),
-    mTextZoom(1.0), mFullZoom(1.0), mPageSize(-1, -1), mPPScale(1.0f),
+    mTextZoom(1.0), mFullZoom(1.0), mLastFontInflationScreenWidth(-1.0),
+    mPageSize(-1, -1), mPPScale(1.0f),
     mViewportStyleOverflow(NS_STYLE_OVERFLOW_AUTO, NS_STYLE_OVERFLOW_AUTO),
     mImageAnimationModePref(imgIContainer::kNormalAnimMode)
 {
@@ -1412,6 +1413,34 @@ nsPresContext::SetFullZoom(float aZoom)
   mCurAppUnitsPerDevPixel = AppUnitsPerDevPixel();
 }
 
+float
+nsPresContext::ScreenWidthInchesForFontInflation(bool* aChanged)
+{
+  if (aChanged) {
+    *aChanged = false;
+  }
+
+  nsDeviceContext *dx = DeviceContext();
+  nsRect clientRect;
+  dx->GetClientRect(clientRect); // FIXME: GetClientRect looks expensive
+  float deviceWidthInches =
+    float(clientRect.width) / float(dx->AppUnitsPerPhysicalInch());
+
+  if (deviceWidthInches != mLastFontInflationScreenWidth) {
+    if (mLastFontInflationScreenWidth != -1.0) {
+      if (aChanged) {
+        *aChanged = true;
+      } else {
+        NS_NOTREACHED("somebody should have checked for screen width change "
+                      "and triggered a reflow");
+      }
+    }
+    mLastFontInflationScreenWidth = deviceWidthInches;
+  }
+
+  return deviceWidthInches;
+}
+
 void
 nsPresContext::SetImageLoaders(nsIFrame* aTargetFrame,
                                ImageLoadType aType,
diff --git a/layout/base/nsPresContext.h b/layout/base/nsPresContext.h
index 59108755842e..5945a87408b7 100644
--- a/layout/base/nsPresContext.h
+++ b/layout/base/nsPresContext.h
@@ -563,7 +563,18 @@ public:
   nscoord GetAutoQualityMinFontSize() {
     return DevPixelsToAppUnits(mAutoQualityMinFontSizePixelsPref);
   }
-  
+
+  /**
+   * Return the device's screen width in inches, for font size
+   * inflation.
+   *
+   * Set |aChanged| to true if the result returned is different from a
+   * previous call to this method.  If a caller passes |aChanged| as
+   * null, then something else must have already checked whether there
+   * was a change (by calling this method).
+   */
+  float ScreenWidthInchesForFontInflation(bool* aChanged = nsnull);
+
   static PRInt32 AppUnitsPerCSSPixel() { return nsDeviceContext::AppUnitsPerCSSPixel(); }
   PRUint32 AppUnitsPerDevPixel() const  { return mDeviceContext->AppUnitsPerDevPixel(); }
   static PRInt32 AppUnitsPerCSSInch() { return nsDeviceContext::AppUnitsPerCSSInch(); }
@@ -1143,6 +1154,8 @@ protected:
   float                 mTextZoom;      // Text zoom, defaults to 1.0
   float                 mFullZoom;      // Page zoom, defaults to 1.0
 
+  float                 mLastFontInflationScreenWidth;
+
   PRInt32               mCurAppUnitsPerDevPixel;
   PRInt32               mAutoQualityMinFontSizePixelsPref;