Issue #1606 - Add support for multi-monitor DPI awareness v2 (W10 1706+)

3 files changed, 32 insertions, 11 deletions

diff --git a/widget/windows/WinUtils.cpp b/widget/windows/WinUtils.cpp
index 28033193fc..a43c92b699 100644
--- a/widget/windows/WinUtils.cpp
+++ b/widget/windows/WinUtils.cpp
@@ -449,10 +449,20 @@ WinUtils::Initialize()
   if (IsWin10OrLater()) {
     HMODULE user32Dll = ::GetModuleHandleW(L"user32");
     if (user32Dll) {
-      sEnableNonClientDpiScaling = (EnableNonClientDpiScalingProc)
-        ::GetProcAddress(user32Dll, "EnableNonClientDpiScaling");
-      sSetThreadDpiAwarenessContext = (SetThreadDpiAwarenessContextProc)
-        ::GetProcAddress(user32Dll, "SetThreadDpiAwarenessContext");
+      auto getThreadDpiAwarenessContext = (decltype(GetThreadDpiAwarenessContext)*)
+        ::GetProcAddress(user32Dll, "GetThreadDpiAwarenessContext");
+      auto areDpiAwarenessContextsEqual = (decltype(AreDpiAwarenessContextsEqual)*)
+        ::GetProcAddress(user32Dll, "AreDpiAwarenessContextsEqual");
+      if (getThreadDpiAwarenessContext && areDpiAwarenessContextsEqual &&
+          areDpiAwarenessContextsEqual(getThreadDpiAwarenessContext(),
+                                       DPI_AWARENESS_CONTEXT_PER_MONITOR_AWARE)) {
+        // Only per-monitor DPI awareness v1 requires these workarounds.
+        // Since this was introduced in Win 8.1 we'll need to keep this indefinitely.
+        sEnableNonClientDpiScaling = (EnableNonClientDpiScalingProc)
+          ::GetProcAddress(user32Dll, "EnableNonClientDpiScaling");
+        sSetThreadDpiAwarenessContext = (SetThreadDpiAwarenessContextProc)
+          ::GetProcAddress(user32Dll, "SetThreadDpiAwarenessContext");
+      }
     }
   }
 
@@ -468,7 +478,7 @@ LRESULT WINAPI
 WinUtils::NonClientDpiScalingDefWindowProcW(HWND hWnd, UINT msg,
                                             WPARAM wParam, LPARAM lParam)
 {
-  if (msg == WM_NCCREATE && sEnableNonClientDpiScaling) {
+  if (msg == WM_NCCREATE) {
     sEnableNonClientDpiScaling(hWnd);
   }
   return ::DefWindowProcW(hWnd, msg, wParam, lParam);
diff --git a/widget/windows/WinUtils.h b/widget/windows/WinUtils.h
index 1281171be6..f268f907fc 100644
--- a/widget/windows/WinUtils.h
+++ b/widget/windows/WinUtils.h
@@ -96,6 +96,11 @@ typedef enum DPI_AWARENESS {
 #define DPI_AWARENESS_CONTEXT_DECLARED
 #endif // (DPI_AWARENESS_CONTEXT_DECLARED)
 
+#if WINVER < 0x0605
+WINUSERAPI DPI_AWARENESS_CONTEXT WINAPI GetThreadDpiAwarenessContext();
+WINUSERAPI BOOL WINAPI
+AreDpiAwarenessContextsEqual(DPI_AWARENESS_CONTEXT, DPI_AWARENESS_CONTEXT);
+#endif /* WINVER < 0x0605 */
 typedef DPI_AWARENESS_CONTEXT(WINAPI * SetThreadDpiAwarenessContextProc)(DPI_AWARENESS_CONTEXT);
 typedef BOOL(WINAPI * EnableNonClientDpiScalingProc)(HWND);
 
@@ -152,6 +157,12 @@ class WinUtils
   static SetThreadDpiAwarenessContextProc sSetThreadDpiAwarenessContext;
   static EnableNonClientDpiScalingProc sEnableNonClientDpiScaling;
 
+  // Wrapper for DefWindowProc that will enable non-client dpi scaling on the
+  // window during creation.
+  static LRESULT WINAPI
+  NonClientDpiScalingDefWindowProcW(HWND hWnd, UINT msg,
+                                    WPARAM wParam, LPARAM lParam);
+
 public:
   class AutoSystemDpiAware
   {
@@ -174,11 +185,11 @@ public:
     DPI_AWARENESS_CONTEXT mPrevContext;
   };
 
-  // Wrapper for DefWindowProc that will enable non-client dpi scaling on the
-  // window during creation.
-  static LRESULT WINAPI
-  NonClientDpiScalingDefWindowProcW(HWND hWnd, UINT msg,
-                                    WPARAM wParam, LPARAM lParam);
+  static decltype(::DefWindowProcW)* GetDefWindowProc() {
+    return sEnableNonClientDpiScaling ?
+           NonClientDpiScalingDefWindowProcW :
+           ::DefWindowProcW;
+  }
 
   /**
    * Get the system's default logical-to-physical DPI scaling factor,
diff --git a/widget/windows/nsWindow.cpp b/widget/windows/nsWindow.cpp
index 907d8b9a5b..b9c3ea1548 100644
--- a/widget/windows/nsWindow.cpp
+++ b/widget/windows/nsWindow.cpp
@@ -999,7 +999,7 @@ nsWindow::RegisterWindowClass(const wchar_t* aClassName,
   }
 
   wc.style         = CS_DBLCLKS | aExtraStyle;
-  wc.lpfnWndProc   = WinUtils::NonClientDpiScalingDefWindowProcW;
+  wc.lpfnWndProc   = WinUtils::GetDefWindowProc();
   wc.cbClsExtra    = 0;
   wc.cbWndExtra    = 0;
   wc.hInstance     = nsToolkit::mDllInstance;