Adds Support for system dpi awareness for older versions of Windows

shell/platform/windows/win32_dpi_helper.cc

@@ -18,7 +18,6 @@ Win32DpiHelper::Win32DpiHelper() {
   if (user32_module_ == nullptr) {
     return;
   }
-
   if (!AssignProcAddress(user32_module_, "EnableNonClientDpiScaling",
                          enable_non_client_dpi_scaling_)) {
     return;
@@ -33,7 +32,6 @@ Win32DpiHelper::Win32DpiHelper() {
                          set_process_dpi_awareness_context_)) {
     return;
   }
-
   permonitorv2_supported_ = true;
 }
 
@@ -43,6 +41,20 @@ Win32DpiHelper::~Win32DpiHelper() {
   }
 }
 
+BOOL Win32DpiHelper::SetDpiAwareness() {
+  if (permonitorv2_supported_) {
+    return set_process_dpi_awareness_context_(
+        DPI_AWARENESS_CONTEXT_PER_MONITOR_AWARE_V2);
+
+  } else {
+    AssignProcAddress(user32_module_, "GetDpiForSystem", get_dpi_for_system_);
+    AssignProcAddress(user32_module_, "SetProcessDpiAware",
+                      set_process_dpi_aware_);
+
+    return set_process_dpi_aware_();
+  }
+}
+
 bool Win32DpiHelper::IsPerMonitorV2Available() {
   return permonitorv2_supported_;
 }
@@ -54,19 +66,11 @@ BOOL Win32DpiHelper::EnableNonClientDpiScaling(HWND hwnd) {
   return enable_non_client_dpi_scaling_(hwnd);
 }
 
-UINT Win32DpiHelper::GetDpiForWindow(HWND hwnd) {
-  if (!permonitorv2_supported_) {
-    return false;
-  }
-  return get_dpi_for_window_(hwnd);
-}
-
-BOOL Win32DpiHelper::SetProcessDpiAwarenessContext(
-    DPI_AWARENESS_CONTEXT context) {
-  if (!permonitorv2_supported_) {
-    return false;
+UINT Win32DpiHelper::GetDpi(HWND hwnd) {
+  if (permonitorv2_supported_) {
+    return get_dpi_for_window_(hwnd);
   }
-  return set_process_dpi_awareness_context_(context);
+  return get_dpi_for_system_();
 }
 
 }  // namespace flutter

shell/platform/windows/win32_dpi_helper.h

@@ -25,20 +25,29 @@ class Win32DpiHelper {
   // Wrapper for OS functionality to turn on automatic window non-client scaling
   BOOL EnableNonClientDpiScaling(HWND);
 
-  // Wrapper for OS functionality to return the DPI for |HWND|
-  UINT GetDpiForWindow(HWND);
+  // Wrapper for OS functionality to return the DPI for |HWND| if Per Monitor V2
+  // awareness has been set. Otherwise, returns the DPI for the System.
+  UINT GetDpi(HWND);
 
-  // Sets the current process to a specified DPI awareness context.
-  BOOL SetProcessDpiAwarenessContext(DPI_AWARENESS_CONTEXT);
+  // Sets the DPI awareness for the application. For versions >= Windows 1703,
+  // use Per Monitor V2. For any older versions, use System.
+  //
+  // This call is overriden if DPI awareness is stated in the application
+  // manifest.
+  BOOL SetDpiAwareness();
 
  private:
   using EnableNonClientDpiScaling_ = BOOL __stdcall(HWND);
   using GetDpiForWindow_ = UINT __stdcall(HWND);
   using SetProcessDpiAwarenessContext_ = BOOL __stdcall(DPI_AWARENESS_CONTEXT);
+  using SetProcessDpiAware_ = BOOL __stdcall();
+  using GetDpiForSystem_ = UINT __stdcall();
 
   EnableNonClientDpiScaling_* enable_non_client_dpi_scaling_ = nullptr;
   GetDpiForWindow_* get_dpi_for_window_ = nullptr;
   SetProcessDpiAwarenessContext_* set_process_dpi_awareness_context_ = nullptr;
+  SetProcessDpiAware_* set_process_dpi_aware_ = nullptr;
+  GetDpiForSystem_* get_dpi_for_system_ = nullptr;
 
   HMODULE user32_module_ = nullptr;
   bool permonitorv2_supported_ = false;

shell/platform/windows/win32_window.cc

@@ -6,16 +6,8 @@
 
 namespace flutter {
 
-Win32Window::Win32Window() {
-  // Assume Windows 10 1703 or greater for DPI handling.  When running on a
-  // older release of Windows where this context doesn't exist, DPI calls will
-  // fail and Flutter rendering will be impacted until this is fixed.
-  // To handle downlevel correctly, dpi_helper must use the most recent DPI
-  // context available should be used: Windows 1703: Per-Monitor V2, 8.1:
-  // Per-Monitor V1, Windows 7: System See
-  // https://docs.microsoft.com/en-us/windows/win32/hidpi/high-dpi-desktop-application-development-on-windows
-  // for more information.
-}
+Win32Window::Win32Window() {}
+
 Win32Window::~Win32Window() {
   Destroy();
 }
@@ -26,7 +18,13 @@ void Win32Window::InitializeChild(const char* title,
   Destroy();
   std::wstring converted_title = NarrowToWide(title);
 
-  WNDCLASS window_class = ResgisterWindowClass(converted_title);
+  // Set DPI awareness for all Windows versions. This call has to be made before
+  // the HWND is created.
+  if (!dpi_helper_->SetDpiAwareness()) {
+    OutputDebugString(L"Failed to set DPI awareness");
+  }
+
+  WNDCLASS window_class = RegisterWindowClass(converted_title);
 
   auto* result = CreateWindowEx(
       0, window_class.lpszClassName, converted_title.c_str(),
@@ -54,7 +52,7 @@ std::wstring Win32Window::NarrowToWide(const char* source) {
   return wideTitle;
 }
 
-WNDCLASS Win32Window::ResgisterWindowClass(std::wstring& title) {
+WNDCLASS Win32Window::RegisterWindowClass(std::wstring& title) {
   window_class_name_ = title;
 
   WNDCLASS window_class{};
@@ -83,13 +81,16 @@ LRESULT CALLBACK Win32Window::WndProc(HWND const window,
 
     auto that = static_cast<Win32Window*>(cs->lpCreateParams);
 
-    // Since the application is running in Per-monitor V2 mode, turn on
-    // automatic titlebar scaling
-    BOOL result = that->dpi_helper_->EnableNonClientDpiScaling(window);
-    if (result != TRUE) {
-      OutputDebugString(L"Failed to enable non-client area autoscaling");
+    if (that->dpi_helper_->IsPerMonitorV2Available()) {
+      // Since the application is running in Per-monitor V2 mode, turn on
+      // automatic titlebar scaling
+      BOOL result = that->dpi_helper_->EnableNonClientDpiScaling(window);
+      if (result != TRUE) {
+        OutputDebugString(L"Failed to enable non-client area autoscaling");
+      }
     }
-    that->current_dpi_ = that->dpi_helper_->GetDpiForWindow(window);
+    that->current_dpi_ = that->dpi_helper_->GetDpi(window);
+
     that->window_handle_ = window;
   } else if (Win32Window* that = GetThisFromHandle(window)) {
     return that->MessageHandler(window, message, wparam, lparam);
@@ -282,7 +283,7 @@ Win32Window::HandleDpiChange(HWND hwnd,
     // The DPI is only passed for DPI change messages on top level windows,
     // hence call function to get DPI if needed.
     if (uDpi == 0) {
-      uDpi = dpi_helper_->GetDpiForWindow(hwnd);
+      uDpi = dpi_helper_->GetDpi(hwnd);
     }
     current_dpi_ = uDpi;
     window->OnDpiScale(uDpi);

shell/platform/windows/win32_window.h

@@ -57,7 +57,7 @@ class Win32Window {
 
   // Registers a window class with default style attributes, cursor and
   // icon.
-  WNDCLASS ResgisterWindowClass(std::wstring& title);
+  WNDCLASS RegisterWindowClass(std::wstring& title);
 
   // OS callback called by message pump.  Handles the WM_NCCREATE message which
   // is passed when the non-client area is being created and enables automatic