bug 859266 - respect devPixelsPerPx preference when calculating screen dimens on Windows. r=roc

widget/nsIWidget.h

@@ -607,6 +607,19 @@ class nsIWidget : public nsISupports {
      */
     double GetDefaultScale();
 
+    /**
+     * Return the Gecko override of the system default scale, if any;
+     * returns <= 0.0 if the system scale should be used as-is.
+     * nsIWidget::GetDefaultScale() [above] takes this into account.
+     * It is exposed here so that code that wants to check for a
+     * default-scale override without having a widget on hand can
+     * easily access the same value.
+     * Note that any scale override is a browser-wide value, whereas
+     * the default GetDefaultScale value (when no override is present)
+     * may vary between widgets (or screens).
+     */
+    static double DefaultScaleOverride();
+
     /**
      * Return the first child of this widget.  Will return null if
      * there are no children.

widget/windows/nsNativeThemeWin.cpp

@@ -202,7 +202,10 @@ GetGutterSize(HANDLE theme, HDC hdc)
     GetThemePartSize(theme, hdc, MENU_POPUPITEM, MPI_NORMAL, NULL, TS_TRUE, &itemSize);
 
     // Figure out how big the menuitem's icon will be (if present) at current DPI
-    FLOAT scaleFactor = gfxWindowsPlatform::GetPlatform()->GetDPIScale();
+    double scaleFactor = nsIWidget::DefaultScaleOverride();
+    if (scaleFactor <= 0.0) {
+      scaleFactor = gfxWindowsPlatform::GetPlatform()->GetDPIScale();
+    }
     int iconDevicePixels = NSToIntRound(16 * scaleFactor);
     SIZE iconSize = {
       iconDevicePixels, iconDevicePixels

widget/windows/nsScreenManagerWin.cpp

@@ -6,6 +6,7 @@
 #include "nsScreenManagerWin.h"
 #include "nsScreenWin.h"
 #include "gfxWindowsPlatform.h"
+#include "nsIWidget.h"
 
 
 BOOL CALLBACK CountMonitors ( HMONITOR, HDC, LPRECT, LPARAM ioCount ) ;
@@ -78,7 +79,10 @@ nsScreenManagerWin :: ScreenForRect ( int32_t inLeft, int32_t inTop, int32_t inW
   }
 
   // convert coordinates from logical to device pixels for MonitorFromRect
-  FLOAT dpiScale = gfxWindowsPlatform::GetPlatform()->GetDPIScale();
+  double dpiScale = nsIWidget::DefaultScaleOverride();
+  if (dpiScale <= 0.0) {
+    dpiScale = gfxWindowsPlatform::GetPlatform()->GetDPIScale(); 
+  }
   RECT globalWindowBounds = {
     NSToIntRound(dpiScale * inLeft),
     NSToIntRound(dpiScale * inTop),

widget/windows/nsScreenWin.cpp

@@ -6,6 +6,7 @@
 #include "nsScreenWin.h"
 #include "nsCoord.h"
 #include "gfxWindowsPlatform.h"
+#include "nsIWidget.h"
 
 
 nsScreenWin :: nsScreenWin ( HMONITOR inScreen )
@@ -89,6 +90,16 @@ nsScreenWin :: GetAvailRect(int32_t *outLeft, int32_t *outTop, int32_t *outWidth
   
 } // GetAvailRect
 
+static double
+GetDPIScale()
+{
+  double dpiScale= nsIWidget::DefaultScaleOverride();
+  if (dpiScale <= 0.0) {
+    dpiScale = gfxWindowsPlatform::GetPlatform()->GetDPIScale(); 
+  }
+  return dpiScale;
+}
+
 NS_IMETHODIMP
 nsScreenWin::GetRectDisplayPix(int32_t *outLeft,  int32_t *outTop,
                                int32_t *outWidth, int32_t *outHeight)
@@ -98,7 +109,7 @@ nsScreenWin::GetRectDisplayPix(int32_t *outLeft,  int32_t *outTop,
   if (NS_FAILED(rv)) {
     return rv;
   }
-  double scaleFactor = 1.0 / gfxWindowsPlatform::GetPlatform()->GetDPIScale(); 
+  double scaleFactor = 1.0 / GetDPIScale();
   *outLeft = NSToIntRound(left * scaleFactor);
   *outTop = NSToIntRound(top * scaleFactor);
   *outWidth = NSToIntRound(width * scaleFactor);
@@ -115,7 +126,7 @@ nsScreenWin::GetAvailRectDisplayPix(int32_t *outLeft,  int32_t *outTop,
   if (NS_FAILED(rv)) {
     return rv;
   }
-  double scaleFactor = 1.0 / gfxWindowsPlatform::GetPlatform()->GetDPIScale(); 
+  double scaleFactor = 1.0 / GetDPIScale();
   *outLeft = NSToIntRound(left * scaleFactor);
   *outTop = NSToIntRound(top * scaleFactor);
   *outWidth = NSToIntRound(width * scaleFactor);

widget/windows/nsWindow.cpp

@@ -1632,7 +1632,7 @@ NS_METHOD nsWindow::ConstrainPosition(bool aAllowSlop,
   if (!mIsTopWidgetWindow) // only a problem for top-level windows
     return NS_OK;
 
-  float dpiScale = gfxWindowsPlatform::GetPlatform()->GetDPIScale();
+  double dpiScale = GetDefaultScale();
 
   // we need to use the window size in logical screen pixels
   int32_t logWidth = std::max<int32_t>(NSToIntRound(mBounds.width / dpiScale), 1);

widget/xpwidgets/nsBaseWidget.cpp

@@ -380,6 +380,18 @@ float nsBaseWidget::GetDPI()
 }
 
 double nsIWidget::GetDefaultScale()
+{
+  double devPixelsPerCSSPixel = DefaultScaleOverride();
+
+  if (devPixelsPerCSSPixel <= 0.0) {
+    devPixelsPerCSSPixel = GetDefaultScaleInternal();
+  }
+
+  return devPixelsPerCSSPixel;
+}
+
+/* static */
+double nsIWidget::DefaultScaleOverride()
 {
   // 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
@@ -391,10 +403,6 @@ double nsIWidget::GetDefaultScale()
     devPixelsPerCSSPixel = PR_strtod(prefString, nullptr);
   }
 
-  if (devPixelsPerCSSPixel <= 0.0) {
-    devPixelsPerCSSPixel = GetDefaultScaleInternal();
-  }
-
   return devPixelsPerCSSPixel;
 }