Bug 804062. Move device-pixels-per-CSS-pixel preference logic into nsIWidget::GetDefaultScale. r=jfkthame

The only behavior change caused by this patch should be that nsDeviceContexts with no widget
(mainly printing, I think), default to a devpixel-per-CSS-pixel ratio of 1.0, ignoring
any prefs set.

gfx/src/nsDeviceContext.cpp

@@ -337,23 +337,7 @@ nsDeviceContext::SetDPI()
             dpi = 96.0f;
         }
 
-        // The number of device pixels per CSS pixel. A value <= 0 means choose
-        // automatically based on the DPI. A positive value is used as-is. This effectively
-        // controls the size of a CSS "px".
-        float devPixelsPerCSSPixel = -1.0;
-
-        nsAdoptingCString prefString = Preferences::GetCString("layout.css.devPixelsPerPx");
-        if (!prefString.IsEmpty()) {
-            devPixelsPerCSSPixel = static_cast<float>(atof(prefString));
-        }
-
-        if (devPixelsPerCSSPixel <= 0) {
-            if (mWidget) {
-                devPixelsPerCSSPixel = mWidget->GetDefaultScale();
-            } else {
-                devPixelsPerCSSPixel = 1.0;
-            }
-        }
+        double devPixelsPerCSSPixel = mWidget ? mWidget->GetDefaultScale() : 1.0;
 
         mAppUnitsPerDevNotScaledPixel =
             NS_MAX(1, NS_lround(AppUnitsPerCSSPixel() / devPixelsPerCSSPixel));

widget/cocoa/nsChildView.h

@@ -408,7 +408,7 @@ public:
   // between HiDPI and non-HiDPI screens
   void                    BackingScaleFactorChanged();
 
-  virtual double          GetDefaultScale();
+  virtual double          GetDefaultScaleInternal();
 
   NS_IMETHOD              Invalidate(const nsIntRect &aRect);
 

widget/cocoa/nsChildView.mm

@@ -742,7 +742,7 @@ NS_IMETHODIMP nsChildView::GetBounds(nsIntRect &aRect)
 }
 
 double
-nsChildView::GetDefaultScale()
+nsChildView::GetDefaultScaleInternal()
 {
   return BackingScaleFactor();
 }

widget/cocoa/nsCocoaWindow.h

@@ -247,7 +247,7 @@ public:
 
     CGFloat                 BackingScaleFactor();
     void                    BackingScaleFactorChanged();
-    virtual double          GetDefaultScale();
+    virtual double          GetDefaultScaleInternal();
 
     NS_IMETHOD              SetTitle(const nsAString& aTitle);
 

widget/cocoa/nsCocoaWindow.mm

@@ -1399,7 +1399,7 @@ NS_IMETHODIMP nsCocoaWindow::GetScreenBounds(nsIntRect &aRect)
 }
 
 double
-nsCocoaWindow::GetDefaultScale()
+nsCocoaWindow::GetDefaultScaleInternal()
 {
   return BackingScaleFactor();
 }

widget/nsIWidget.h

@@ -90,8 +90,8 @@ typedef nsEventStatus (* EVENT_CALLBACK)(nsGUIEvent *event);
 #endif
 
 #define NS_IWIDGET_IID \
-  { 0x8181a08f, 0xaa37, 0x4fd0, \
-    { 0x94, 0x32, 0x27, 0x74, 0xe2, 0xce, 0x02, 0xc8 } }
+  { 0xb7c60bda, 0xe16c, 0x4e89, \
+    { 0x86, 0x8c, 0xc3, 0x2e, 0x62, 0x40, 0x05, 0xb2 } }
 
 /*
  * Window shadow styles
@@ -574,9 +574,10 @@ class nsIWidget : public nsISupports {
      * Return the default scale factor for the window. This is the
      * default number of device pixels per CSS pixel to use. This should
      * depend on OS/platform settings such as the Mac's "UI scale factor"
-     * or Windows' "font DPI".
+     * or Windows' "font DPI". This will take into account Gecko preferences
+     * overriding the system setting.
      */
-    virtual double GetDefaultScale() = 0;
+    double GetDefaultScale();
 
     /**
      * Return the first child of this widget.  Will return null if
@@ -1652,6 +1653,11 @@ class nsIWidget : public nsISupports {
     { return nullptr; }
 
 protected:
+    /**
+     * Like GetDefaultScale, but taking into account only the system settings
+     * and ignoring Gecko preferences.
+     */
+    virtual double GetDefaultScaleInternal() { return 1.0; }
 
     // keep the list of children.  We also keep track of our siblings.
     // The ownership model is as follows: parent holds a strong ref to

widget/windows/nsWindow.cpp

@@ -975,7 +975,7 @@ float nsWindow::GetDPI()
   return float(heightPx/heightInches);
 }
 
-double nsWindow::GetDefaultScale()
+double nsWindow::GetDefaultScaleInternal()
 {
   HDC dc = ::GetDC(mWnd);
   if (!dc)

widget/windows/nsWindow.h

@@ -90,7 +90,7 @@ public:
   NS_IMETHOD              SetParent(nsIWidget *aNewParent);
   virtual nsIWidget*      GetParent(void);
   virtual float           GetDPI();
-  virtual double          GetDefaultScale();
+  virtual double          GetDefaultScaleInternal();
   NS_IMETHOD              Show(bool bState);
   virtual bool            IsVisible() const;
   NS_IMETHOD              ConstrainPosition(bool aAllowSlop, int32_t *aX, int32_t *aY);

widget/xpwidgets/nsBaseWidget.cpp

@@ -377,9 +377,23 @@ float nsBaseWidget::GetDPI()
   return 96.0f;
 }
 
-double nsBaseWidget::GetDefaultScale()
+double nsIWidget::GetDefaultScale()
 {
-  return 1.0;
+  // The number of device pixels per CSS pixel. A value <= 0 means choose
+  // automatically based on the DPI. A positive value is used as-is. This effectively
+  // controls the size of a CSS "px".
+  float devPixelsPerCSSPixel = -1.0;
+
+  nsAdoptingCString prefString = Preferences::GetCString("layout.css.devPixelsPerPx");
+  if (!prefString.IsEmpty()) {
+    devPixelsPerCSSPixel = static_cast<float>(atof(prefString));
+  }
+
+  if (devPixelsPerCSSPixel <= 0) {
+    devPixelsPerCSSPixel = GetDefaultScaleInternal();
+  }
+
+  return devPixelsPerCSSPixel;
 }
 
 //-------------------------------------------------------------------------

widget/xpwidgets/nsBaseWidget.h

@@ -72,7 +72,6 @@ public:
   virtual nsIWidget*      GetTopLevelWidget();
   virtual nsIWidget*      GetSheetWindowParent(void);
   virtual float           GetDPI();
-  virtual double          GetDefaultScale();
   virtual void            AddChild(nsIWidget* aChild);
   virtual void            RemoveChild(nsIWidget* aChild);