Bug 1302168: Fix maximized caption height with per-monitor DPI r=jmathies

Differential Revision: https://phabricator.services.mozilla.com/D4957

--HG--
extra : moz-landing-system : lando

widget/windows/WinUtils.cpp

@@ -433,6 +433,7 @@ struct CoTaskMemFreePolicy
 
 SetThreadDpiAwarenessContextProc WinUtils::sSetThreadDpiAwarenessContext = NULL;
 EnableNonClientDpiScalingProc WinUtils::sEnableNonClientDpiScaling = NULL;
+GetSystemMetricsForDpiProc WinUtils::sGetSystemMetricsForDpi = NULL;
 
 /* static */
 void
@@ -454,6 +455,9 @@ WinUtils::Initialize()
         sSetThreadDpiAwarenessContext = (SetThreadDpiAwarenessContextProc)
           ::GetProcAddress(user32Dll, "SetThreadDpiAwarenessContext");
       }
+
+      sGetSystemMetricsForDpi = (GetSystemMetricsForDpiProc)
+        ::GetProcAddress(user32Dll, "GetSystemMetricsForDpi");
     }
   }
 }
@@ -671,6 +675,27 @@ WinUtils::MonitorFromRect(const gfx::Rect& rect)
   return ::MonitorFromRect(&globalWindowBounds, MONITOR_DEFAULTTONEAREST);
 }
 
+/* static */
+bool
+WinUtils::HasSystemMetricsForDpi()
+{
+  return (sGetSystemMetricsForDpi != NULL);
+}
+
+/* static */
+int
+WinUtils::GetSystemMetricsForDpi(int nIndex, UINT dpi)
+{
+  if (HasSystemMetricsForDpi()) {
+    return sGetSystemMetricsForDpi(nIndex, dpi);
+  } else {
+    double scale = IsPerMonitorDPIAware()
+      ? dpi / SystemDPI()
+      : 1.0;
+    return NSToIntRound(::GetSystemMetrics(nIndex) * scale);
+  }
+}
+
 #ifdef ACCESSIBILITY
 /* static */
 a11y::Accessible*

widget/windows/WinUtils.h

@@ -103,6 +103,7 @@ AreDpiAwarenessContextsEqual(DPI_AWARENESS_CONTEXT, DPI_AWARENESS_CONTEXT);
 #endif /* WINVER < 0x0605 */
 typedef DPI_AWARENESS_CONTEXT(WINAPI * SetThreadDpiAwarenessContextProc)(DPI_AWARENESS_CONTEXT);
 typedef BOOL(WINAPI * EnableNonClientDpiScalingProc)(HWND);
+typedef int (WINAPI * GetSystemMetricsForDpiProc)(int, UINT);
 
 namespace mozilla {
 enum class PointerCapabilities : uint8_t;
@@ -163,6 +164,7 @@ class WinUtils
   // the Win10 update version -- will be set up in Initialize().
   static SetThreadDpiAwarenessContextProc sSetThreadDpiAwarenessContext;
   static EnableNonClientDpiScalingProc sEnableNonClientDpiScaling;
+  static GetSystemMetricsForDpiProc sGetSystemMetricsForDpi;
 
 public:
   class AutoSystemDpiAware
@@ -227,6 +229,9 @@ public:
   static HMONITOR GetPrimaryMonitor();
   static HMONITOR MonitorFromRect(const gfx::Rect& rect);
 
+  static bool HasSystemMetricsForDpi();
+  static int GetSystemMetricsForDpi(int nIndex, UINT dpi);
+
   /**
    * Logging helpers that dump output to prlog module 'Widget', console, and
    * OutputDebugString. Note these output in both debug and release builds.

widget/windows/nsNativeThemeWin.cpp

@@ -2168,9 +2168,20 @@ nsNativeThemeWin::GetWidgetPadding(nsDeviceContext* aContext,
     // XXX Maximized windows have an offscreen offset equal to
     // the border padding. This should be addressed in nsWindow,
     // but currently can't be, see UpdateNonClientMargins.
-    if (aWidgetType == StyleAppearance::MozWindowTitlebarMaximized)
-      aResult->top = GetSystemMetrics(SM_CXFRAME)
-                   + GetSystemMetrics(SM_CXPADDEDBORDER);
+    if (aWidgetType == StyleAppearance::MozWindowTitlebarMaximized) {
+      nsIWidget* rootWidget = nullptr;
+      if (WinUtils::HasSystemMetricsForDpi()) {
+        rootWidget = aFrame->PresContext()->GetRootWidget();
+      }
+      if (rootWidget) {
+        double dpi = rootWidget->GetDPI();
+        aResult->top = WinUtils::GetSystemMetricsForDpi(SM_CXFRAME, dpi)
+                     + WinUtils::GetSystemMetricsForDpi(SM_CXPADDEDBORDER, dpi);
+      } else {
+        aResult->top = GetSystemMetrics(SM_CXFRAME)
+                     + GetSystemMetrics(SM_CXPADDEDBORDER);
+      }
+    }
     return ok;
   }