Backed out changeset 61387758d1c4 (bug 1376931)

layout/style/nsMediaFeatures.cpp

@@ -279,24 +279,16 @@ static void
 GetResolution(nsPresContext* aPresContext, const nsMediaFeature*,
               nsCSSValue& aResult)
 {
-  // We're returning resolution in terms of device pixels per css pixel, since
-  // that is the preferred unit for media queries of resolution. This avoids
-  // introducing precision error from conversion to and from less-used
-  // physical units like inches.
-
-  float dppx;
+  float dpi = 96; // Use 96 when resisting fingerprinting.
 
   if (!ShouldResistFingerprinting(aPresContext)) {
-    // Get the actual device pixel ratio, which also takes zoom into account.
-    dppx = float(nsPresContext::AppUnitsPerCSSPixel()) /
-             aPresContext->AppUnitsPerDevPixel();
-  } else {
-    // We are resisting fingerprinting, so pretend we have a device pixel ratio
-    // of 1. In that case, we simply report the zoom level.
-    dppx = aPresContext->GetEffectiveFullZoom();
+    // Resolution measures device pixels per CSS (inch/cm/pixel).  We
+    // return it in device pixels per CSS inches.
+    dpi = float(nsPresContext::AppUnitsPerCSSInch()) /
+          float(aPresContext->AppUnitsPerDevPixel());
   }
 
-  aResult.SetFloatValue(dppx, eCSSUnit_Pixel);
+  aResult.SetFloatValue(dpi, eCSSUnit_Inch);
 }
 
 static void

layout/style/nsMediaList.cpp

@@ -129,23 +129,23 @@ nsMediaExpression::Matches(nsPresContext *aPresContext,
                      required.GetUnit() == eCSSUnit_Pixel ||
                      required.GetUnit() == eCSSUnit_Centimeter,
                      "bad required value");
-        float actualDPPX = actual.GetFloatValue();
+        float actualDPI = actual.GetFloatValue();
         float overrideDPPX = aPresContext->GetOverrideDPPX();
 
         if (overrideDPPX > 0) {
-          actualDPPX = overrideDPPX;
+          actualDPI = overrideDPPX * 96.0f;
         } else if (actual.GetUnit() == eCSSUnit_Centimeter) {
-          actualDPPX = actualDPPX * 2.54f / 96.0f;
-        } else if (actual.GetUnit() == eCSSUnit_Inch) {
-          actualDPPX = actualDPPX / 96.0f;
+          actualDPI = actualDPI * 2.54f;
+        } else if (actual.GetUnit() == eCSSUnit_Pixel) {
+          actualDPI = actualDPI * 96.0f;
         }
-        float requiredDPPX = required.GetFloatValue();
+        float requiredDPI = required.GetFloatValue();
         if (required.GetUnit() == eCSSUnit_Centimeter) {
-          requiredDPPX = requiredDPPX * 2.54f / 96.0f;
-        } else if (required.GetUnit() == eCSSUnit_Inch) {
-          requiredDPPX = requiredDPPX / 96.0f;
+          requiredDPI = requiredDPI * 2.54f;
+        } else if (required.GetUnit() == eCSSUnit_Pixel) {
+          requiredDPI = requiredDPI * 96.0f;
         }
-        cmp = DoCompare(actualDPPX, requiredDPPX);
+        cmp = DoCompare(actualDPI, requiredDPI);
       }
       break;
     case nsMediaFeature::eEnumerated: