bug 851520 part 1 - expose systemDefaultScale attribute on nsIScreenManager. r=roc

widget/android/nsScreenManagerAndroid.cpp

@@ -105,3 +105,9 @@ nsScreenManagerAndroid::GetNumberOfScreens(uint32_t *aNumberOfScreens)
     return NS_OK;
 }
 
+NS_IMETHODIMP
+nsScreenManagerAndroid::GetSystemDefaultScale(float *aDefaultScale)
+{
+    *aDefaultScale = 1.0f;
+    return NS_OK;
+}

widget/cocoa/nsScreenManagerCocoa.mm

@@ -96,6 +96,13 @@ nsScreenManagerCocoa::GetNumberOfScreens (uint32_t *aNumberOfScreens)
     NS_OBJC_END_TRY_ABORT_BLOCK_NSRESULT;
 }
 
+NS_IMETHODIMP
+nsScreenManagerCocoa::GetSystemDefaultScale(float *aDefaultScale)
+{
+    *aDefaultScale = 1.0f;
+    return NS_OK;
+}
+
 NS_IMETHODIMP
 nsScreenManagerCocoa::ScreenForNativeWidget (void *nativeWidget, nsIScreen **outScreen)
 {

widget/gtk2/nsScreenManagerGtk.cpp

@@ -284,6 +284,13 @@ nsScreenManagerGtk :: GetNumberOfScreens(uint32_t *aNumberOfScreens)
   
 } // GetNumberOfScreens
 
+NS_IMETHODIMP
+nsScreenManagerGtk::GetSystemDefaultScale(float *aDefaultScale)
+{
+  *aDefaultScale = 1.0f;
+  return NS_OK;
+}
+
 NS_IMETHODIMP
 nsScreenManagerGtk :: ScreenForNativeWidget (void *aWidget, nsIScreen **outScreen)
 {

widget/nsIScreenManager.idl

@@ -7,7 +7,7 @@
 #include "nsISupports.idl"
 #include "nsIScreen.idl"
 
-[scriptable, uuid(B92319E6-9A84-4ca7-A2CC-EEC22EA9854E)]
+[scriptable, uuid(1C195990-FF9E-412B-AFE7-67D1C660BB27)]
 interface nsIScreenManager : nsISupports
 {
     //
@@ -25,6 +25,26 @@ interface nsIScreenManager : nsISupports
     // Holds the number of screens that are available
   readonly attribute unsigned long numberOfScreens;
 
+    // The default DPI scaling factor of the screen environment (number of
+    // screen pixels corresponding to 1 CSS px, at the default zoom level).
+    //
+    // This is currently fixed at 1.0 on most platforms, but varies on Windows
+    // if the "logical DPI" scaling option in the Display control panel is set
+    // to a value other than 100% (e.g. 125% or 150% are increasingly common
+    // defaults on laptops with high-dpi screens). See bug 851520.
+    //
+    // NOTE that on OS X, this does -not- reflect the "backing scale factor"
+    // used to support Retina displays, which is a per-display property,
+    // not a system-wide scaling factor. The default ratio of CSS pixels to
+    // Cocoa points remains 1:1, even on a Retina screen where one Cocoa point
+    // corresponds to two device pixels. (This is exposed via other APIs:
+    // see window.devicePixelRatio).
+    //
+    // NOTE also that on Linux, this does -not- currently reflect changes
+    // to the system-wide (X11 or Gtk2) DPI value, as Firefox does not yet
+    // honor these settings. See bug 798362 and bug 712898.
+  readonly attribute float systemDefaultScale;
+
     // Returns the nsIScreen instance for the given native widget pointer;
     // the pointer is specific to the particular widget implementation,
     // and is generally of the same type that NS_NATIVE_WINDOW is.

widget/os2/nsScreenManagerOS2.cpp

@@ -91,6 +91,13 @@ nsScreenManagerOS2 :: GetNumberOfScreens(uint32_t *aNumberOfScreens)
   
 } // GetNumberOfScreens
 
+NS_IMETHODIMP
+nsScreenManagerOS2::GetSystemDefaultScale(float *aDefaultScale)
+{
+  *aDefaultScale = 1.0f;
+  return NS_OK;
+}
+
 NS_IMETHODIMP
 nsScreenManagerOS2 :: ScreenForNativeWidget(void *nativeWidget, nsIScreen **aScreen)
 {

widget/qt/nsScreenManagerQt.cpp

@@ -99,6 +99,13 @@ nsScreenManagerQt::GetNumberOfScreens(uint32_t *aNumberOfScreens)
     return NS_OK;
 }
 
+NS_IMETHODIMP
+nsScreenManagerQt::GetSystemDefaultScale(float *aDefaultScale)
+{
+    *aDefaultScale = 1.0f;
+    return NS_OK;
+}
+
 NS_IMETHODIMP
 nsScreenManagerQt :: ScreenForNativeWidget (void *aWidget, nsIScreen **outScreen)
 {

widget/windows/nsScreenManagerWin.cpp

@@ -154,6 +154,13 @@ nsScreenManagerWin :: GetNumberOfScreens(uint32_t *aNumberOfScreens)
   
 } // GetNumberOfScreens
 
+NS_IMETHODIMP
+nsScreenManagerWin::GetSystemDefaultScale(float *aDefaultScale)
+{
+  *aDefaultScale = float(gfxWindowsPlatform::GetPlatform()->GetDPIScale());
+  return NS_OK;
+}
+
 NS_IMETHODIMP
 nsScreenManagerWin :: ScreenForNativeWidget(void *aWidget, nsIScreen **outScreen)
 {

widget/xpwidgets/PuppetWidget.cpp

@@ -762,5 +762,12 @@ PuppetScreenManager::GetNumberOfScreens(uint32_t* aNumberOfScreens)
   return NS_OK;
 }
 
+NS_IMETHODIMP
+PuppetScreenManager::GetSystemDefaultScale(float *aDefaultScale)
+{
+  *aDefaultScale = 1.0f;
+  return NS_OK;
+}
+
 }  // namespace widget
 }  // namespace mozilla