bug 852522 - (win-hidpi) scale system font sizes in response to dpi/devPixelsPerPx setting. r=jimm

widget/windows/nsLookAndFeel.cpp

@@ -555,18 +555,19 @@ GetSysFontInfo(HDC aHDC, LookAndFeel::FontID anID,
     break;
   }
 
-  // FIXME?: mPixelScale is currently hardcoded to 1.
-  float mPixelScale = 1.0f;
+  // Get scaling factor from logical to physical pixels
+  float pixelScale = 96.0 / GetDeviceCaps(aHDC, LOGPIXELSY);
 
   // The lfHeight is in pixels, and it needs to be adjusted for the
   // device it will be displayed on.
   // Screens and Printers will differ in DPI
   //
   // So this accounts for the difference in the DeviceContexts
-  // The mPixelScale will be a "1" for the screen and could be
-  // any value when going to a printer, for example mPixleScale is
+  // The pixelScale will typically be 1.0 for the screen
+  // (though larger for hi-dpi screens where the Windows resolution
+  // scale factor is 125% or 150% or even more), and could be
+  // any value when going to a printer, for example pixelScale is
   // 6.25 when going to a 600dpi printer.
-  // round, but take into account whether it is negative
   float pixelHeight = -ptrLogFont->lfHeight;
   if (pixelHeight < 0) {
     HFONT hFont = ::CreateFontIndirectW(ptrLogFont);
@@ -579,7 +580,7 @@ GetSysFontInfo(HDC aHDC, LookAndFeel::FontID anID,
     ::DeleteObject(hFont);
     pixelHeight = tm.tmAscent;
   }
-  pixelHeight *= mPixelScale;
+  pixelHeight *= pixelScale;
 
   // we have problem on Simplified Chinese system because the system
   // report the default font size is 8 points. but if we use 8, the text
@@ -619,6 +620,8 @@ nsLookAndFeel::GetFontImpl(FontID anID, nsString &aFontName,
   HDC tdc = GetDC(NULL);
   bool status = GetSysFontInfo(tdc, anID, aFontName, aFontStyle);
   ReleaseDC(NULL, tdc);
+  // now convert the logical font size from GetSysFontInfo into device pixels for layout
+  aFontStyle.size *= aDevPixPerCSSPixel;
   return status;
 }