Bug 1376931 Part 3: Change Gecko media queries of resolution to compare in dppx units without unit conversion. r=heycam

MozReview-Commit-ID: DPdz1Tmv24R

--HG--
extra : rebase_source : ddf4b4aae7fdc8ca62de714b9f222daee5959743

layout/style/nsMediaFeatures.cpp

@@ -279,16 +279,24 @@ static void
 GetResolution(nsPresContext* aPresContext, const nsMediaFeature*,
               nsCSSValue& aResult)
 {
-  float dpi = 96; // Use 96 when resisting fingerprinting.
+  // 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;
 
   if (!ShouldResistFingerprinting(aPresContext)) {
-    // 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());
+    // 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();
   }
 
-  aResult.SetFloatValue(dpi, eCSSUnit_Inch);
+  aResult.SetFloatValue(dppx, eCSSUnit_Pixel);
 }
 
 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 actualDPI = actual.GetFloatValue();
+        float actualDPPX = actual.GetFloatValue();
         float overrideDPPX = aPresContext->GetOverrideDPPX();
 
         if (overrideDPPX > 0) {
-          actualDPI = overrideDPPX * 96.0f;
+          actualDPPX = overrideDPPX;
         } else if (actual.GetUnit() == eCSSUnit_Centimeter) {
-          actualDPI = actualDPI * 2.54f;
-        } else if (actual.GetUnit() == eCSSUnit_Pixel) {
-          actualDPI = actualDPI * 96.0f;
+          actualDPPX = actualDPPX * 2.54f / 96.0f;
+        } else if (actual.GetUnit() == eCSSUnit_Inch) {
+          actualDPPX = actualDPPX / 96.0f;
         }
-        float requiredDPI = required.GetFloatValue();
+        float requiredDPPX = required.GetFloatValue();
         if (required.GetUnit() == eCSSUnit_Centimeter) {
-          requiredDPI = requiredDPI * 2.54f;
-        } else if (required.GetUnit() == eCSSUnit_Pixel) {
-          requiredDPI = requiredDPI * 96.0f;
+          requiredDPPX = requiredDPPX * 2.54f / 96.0f;
+        } else if (required.GetUnit() == eCSSUnit_Inch) {
+          requiredDPPX = requiredDPPX / 96.0f;
         }
-        cmp = DoCompare(actualDPI, requiredDPI);
+        cmp = DoCompare(actualDPPX, requiredDPPX);
       }
       break;
     case nsMediaFeature::eEnumerated: