Bug 1242720 - Use (non-dynamic) resolution from GetDeviceCaps when dealing with native-theme code that does not handle dynamic changes to system DPI. r=emk

widget/windows/WinUtils.cpp

@@ -524,6 +524,22 @@ WinUtils::Log(const char *fmt, ...)
   delete[] buffer;
 }
 
+// static
+double
+WinUtils::SystemScaleFactor()
+{
+  // The result of GetDeviceCaps won't change dynamically, as it predates
+  // per-monitor DPI and support for on-the-fly resolution changes.
+  // Therefore, we only need to look it up once.
+  static int logPixelsY = 0;
+  if (!logPixelsY) {
+    HDC screenDC = GetDC(nullptr);
+    logPixelsY = GetDeviceCaps(screenDC, LOGPIXELSY);
+    ReleaseDC(nullptr, screenDC);
+  }
+  return logPixelsY / 96.0;
+}
+
 #ifndef WM_DPICHANGED
 typedef enum {
   MDT_EFFECTIVE_DPI = 0,

widget/windows/WinUtils.h

@@ -128,6 +128,15 @@ public:
 class WinUtils
 {
 public:
+  /**
+   * Get the system's default logical-to-physical DPI scaling factor,
+   * which is based on the primary display. Note however that unlike
+   * LogToPhysFactor(GetPrimaryMonitor()), this will not change during
+   * a session even if the displays are reconfigured. This scale factor
+   * is used by Windows theme metrics etc, which do not fully support
+   * dynamic resolution changes but are only updated on logout.
+   */
+  static double SystemScaleFactor();
 
   static bool IsPerMonitorDPIAware();
   /**

widget/windows/nsLookAndFeel.cpp

@@ -584,7 +584,7 @@ GetSysFontInfo(HDC aHDC, LookAndFeel::FontID anID,
   }
 
   // Get scaling factor from physical to logical pixels
-  double pixelScale = 1.0 / WinUtils::LogToPhysFactor(aHDC);
+  double pixelScale = 1.0 / WinUtils::SystemScaleFactor();
 
   // The lfHeight is in pixels, and it needs to be adjusted for the
   // device it will be displayed on.

widget/windows/nsNativeThemeWin.cpp

@@ -1547,18 +1547,17 @@ AssumeThemePartAndStateAreTransparent(int32_t aPart, int32_t aState)
 }
 
 // When running with per-monitor DPI (on Win8.1+), and rendering on a display
-// with a different DPI setting from the system's primary monitor, we need to
+// with a different DPI setting from the system's default scaling, we need to
 // apply scaling to native-themed elements as the Windows theme APIs assume
-// the primary monitor's resolution.
+// the system default resolution.
 static inline double
 GetThemeDpiScaleFactor(nsIFrame* aFrame)
 {
   if (WinUtils::IsPerMonitorDPIAware() && GetSystemMetrics(SM_CMONITORS) > 1) {
     nsIWidget* rootWidget = aFrame->PresContext()->GetRootWidget();
     if (rootWidget) {
-      double primaryScale =
-        WinUtils::LogToPhysFactor(WinUtils::GetPrimaryMonitor());
-      return rootWidget->GetDefaultScale().scale / primaryScale;
+      double systemScale = WinUtils::SystemScaleFactor();
+      return rootWidget->GetDefaultScale().scale / systemScale;
     }
   }
   return 1.0;