Convert cursor rect to device coordinates on Win32 (flutter/engine#24672)

The handlers for the TextInput.setMarkedTextRect and
TextInput.setEditableSizeAndTransform in the win32 embedding deal in
Flutter root view co-ordinates. These need to be converted to window
co-ordinates before being passed to the TextInputManager, which deals in
Win32 window co-ordinates.

This fixes a bug wherein the IME candidates window for CJK input was
incorrectly positioned at display scales other than 100% in the OS
settings.

Fixes: https://github.com/flutter/flutter/issues/76902

engine/src/flutter/shell/platform/windows/flutter_windows_view.cc

@@ -228,7 +228,7 @@ void FlutterWindowsView::OnScroll(double x,
 }
 
 void FlutterWindowsView::OnCursorRectUpdated(const Rect& rect) {
-  binding_handler_->UpdateCursorRect(rect);
+  binding_handler_->OnCursorRectUpdated(rect);
 }
 
 // Sends new size  information to FlutterEngine.

engine/src/flutter/shell/platform/windows/testing/mock_window_binding_handler.h

@@ -29,7 +29,7 @@ class MockWindowBindingHandler : public WindowBindingHandler {
   MOCK_METHOD0(OnWindowResized, void());
   MOCK_METHOD0(GetPhysicalWindowBounds, PhysicalWindowBounds());
   MOCK_METHOD1(UpdateFlutterCursor, void(const std::string& cursor_name));
-  MOCK_METHOD1(UpdateCursorRect, void(const Rect& rect));
+  MOCK_METHOD1(OnCursorRectUpdated, void(const Rect& rect));
 };
 
 }  // namespace testing

engine/src/flutter/shell/platform/windows/text_input_plugin_delegate.h

@@ -12,7 +12,8 @@ namespace flutter {
 
 class TextInputPluginDelegate {
  public:
-  // Notifies delegate that the cursor position has changed.
+  // Notifies the delegate of the updated the cursor rect in Flutter root view
+  // coordinates.
   virtual void OnCursorRectUpdated(const Rect& rect) = 0;
 };
 

engine/src/flutter/shell/platform/windows/win32_flutter_window.cc

@@ -194,8 +194,12 @@ void Win32FlutterWindow::OnScroll(double delta_x, double delta_y) {
                                       kScrollOffsetMultiplier);
 }
 
-void Win32FlutterWindow::UpdateCursorRect(const Rect& rect) {
-  text_input_manager_.UpdateCaretRect(rect);
+void Win32FlutterWindow::OnCursorRectUpdated(const Rect& rect) {
+  // Convert the rect from Flutter logical coordinates to device coordinates.
+  auto scale = GetDpiScale();
+  Point origin(rect.left() * scale, rect.top() * scale);
+  Size size(rect.width() * scale, rect.height() * scale);
+  UpdateCursorRect(Rect(origin, size));
 }
 
 }  // namespace flutter

engine/src/flutter/shell/platform/windows/win32_flutter_window.h

@@ -71,6 +71,9 @@ class Win32FlutterWindow : public Win32Window, public WindowBindingHandler {
   // |Win32Window|
   void OnComposeChange(const std::u16string& text, int cursor_pos) override;
 
+  // |FlutterWindowBindingHandler|
+  void OnCursorRectUpdated(const Rect& rect) override;
+
   // |Win32Window|
   void OnScroll(double delta_x, double delta_y) override;
 
@@ -89,9 +92,6 @@ class Win32FlutterWindow : public Win32Window, public WindowBindingHandler {
   // |FlutterWindowBindingHandler|
   void UpdateFlutterCursor(const std::string& cursor_name) override;
 
-  // |FlutterWindowBindingHandler|
-  void UpdateCursorRect(const Rect& rect) override;
-
   // |FlutterWindowBindingHandler|
   void OnWindowResized() override;
 

engine/src/flutter/shell/platform/windows/win32_flutter_window_unittests.cc

@@ -21,6 +21,7 @@
 
 using testing::_;
 using testing::Invoke;
+using testing::Return;
 
 namespace flutter {
 namespace testing {
@@ -123,7 +124,10 @@ class SpyTextInputPlugin : public KeyboardHandlerBase,
 
 class MockWin32FlutterWindow : public Win32FlutterWindow {
  public:
-  MockWin32FlutterWindow() : Win32FlutterWindow(800, 600) {}
+  MockWin32FlutterWindow() : Win32FlutterWindow(800, 600) {
+    ON_CALL(*this, GetDpiScale())
+        .WillByDefault(Return(this->Win32FlutterWindow::GetDpiScale()));
+  }
   virtual ~MockWin32FlutterWindow() {}
 
   // Prevent copying.
@@ -149,6 +153,8 @@ class MockWin32FlutterWindow : public Win32FlutterWindow {
   MOCK_METHOD0(OnSetCursor, void());
   MOCK_METHOD2(OnScroll, void(double, double));
   MOCK_METHOD4(DefaultWindowProc, LRESULT(HWND, UINT, WPARAM, LPARAM));
+  MOCK_METHOD0(GetDpiScale, float());
+  MOCK_METHOD1(UpdateCursorRect, void(const Rect&));
 };
 
 // A FlutterWindowsView that overrides the RegisterKeyboardHandlers function
@@ -472,5 +478,34 @@ TEST(Win32FlutterWindowTest, ModifierKeyDownPropagation) {
   }
 }
 
+// Tests that composing rect updates are transformed from Flutter logical
+// coordinates to device coordinates and passed to the text input manager
+// when the DPI scale is 100% (96 DPI).
+TEST(Win32FlutterWindowTest, OnCursorRectUpdatedRegularDPI) {
+  MockWin32FlutterWindow win32window;
+  ON_CALL(win32window, GetDpiScale()).WillByDefault(Return(1.0));
+  EXPECT_CALL(win32window, GetDpiScale()).Times(1);
+
+  Rect cursor_rect(Point(10, 20), Size(30, 40));
+  EXPECT_CALL(win32window, UpdateCursorRect(cursor_rect)).Times(1);
+
+  win32window.OnCursorRectUpdated(cursor_rect);
+}
+
+// Tests that composing rect updates are transformed from Flutter logical
+// coordinates to device coordinates and passed to the text input manager
+// when the DPI scale is 150% (144 DPI).
+TEST(Win32FlutterWindowTest, OnCursorRectUpdatedHighDPI) {
+  MockWin32FlutterWindow win32window;
+  ON_CALL(win32window, GetDpiScale()).WillByDefault(Return(1.5));
+  EXPECT_CALL(win32window, GetDpiScale()).Times(1);
+
+  Rect expected_cursor_rect(Point(15, 30), Size(45, 60));
+  EXPECT_CALL(win32window, UpdateCursorRect(expected_cursor_rect)).Times(1);
+
+  Rect cursor_rect(Point(10, 20), Size(30, 40));
+  win32window.OnCursorRectUpdated(cursor_rect);
+}
+
 }  // namespace testing
 }  // namespace flutter

engine/src/flutter/shell/platform/windows/win32_window.cc

@@ -173,6 +173,10 @@ void Win32Window::OnImeRequest(UINT const message,
   // https://github.com/flutter/flutter/issues/74547
 }
 
+void Win32Window::UpdateCursorRect(const Rect& rect) {
+  text_input_manager_.UpdateCaretRect(rect);
+}
+
 LRESULT
 Win32Window::HandleMessage(UINT const message,
                            WPARAM const wparam,

engine/src/flutter/shell/platform/windows/win32_window.h

@@ -141,6 +141,11 @@ class Win32Window {
                     WPARAM const wparam,
                     LPARAM const lparam);
 
+  // Called when the cursor rect has been updated.
+  //
+  // |rect| is in Win32 window coordinates.
+  virtual void UpdateCursorRect(const Rect& rect);
+
   // Called when mouse scrollwheel input occurs.
   virtual void OnScroll(double delta_x, double delta_y) = 0;
 
@@ -186,7 +191,7 @@ class Win32Window {
   // message.
   int keycode_for_char_message_ = 0;
 
- protected:
+  // Manages IME state.
   TextInputManagerWin32 text_input_manager_;
 };
 

engine/src/flutter/shell/platform/windows/window_binding_handler.h

@@ -63,8 +63,8 @@ class WindowBindingHandler {
   // content. See mouse_cursor.dart for the values and meanings of cursor_name.
   virtual void UpdateFlutterCursor(const std::string& cursor_name) = 0;
 
-  // Sets the cursor rect in root view coordinates.
-  virtual void UpdateCursorRect(const Rect& rect) = 0;
+  // Invoked when the cursor/composing rect has been updated in the framework.
+  virtual void OnCursorRectUpdated(const Rect& rect) = 0;
 };
 
 }  // namespace flutter