Track "mouse leave" event

shell/platform/windows/win32_flutter_window.cc

@@ -125,6 +125,12 @@ void Win32FlutterWindow::OnPointerUp(double x, double y) {
   }
 }
 
+void Win32FlutterWindow::OnPointerLeave() {
+  if (process_events_) {
+    SendPointerLeave();
+  }
+}
+
 void Win32FlutterWindow::OnChar(char32_t code_point) {
   if (process_events_) {
     SendChar(code_point);
@@ -214,6 +220,12 @@ void Win32FlutterWindow::SendPointerUp(double x, double y) {
   SendPointerEventWithData(event);
 }
 
+void Win32FlutterWindow::SendPointerLeave() {
+  FlutterPointerEvent event = {};
+  event.phase = FlutterPointerPhase::kRemove;
+  SendPointerEventWithData(event);
+}
+
 void Win32FlutterWindow::SendChar(char32_t code_point) {
   for (const auto& handler : keyboard_hook_handlers_) {
     handler->CharHook(this, code_point);

shell/platform/windows/win32_flutter_window.h

@@ -53,6 +53,9 @@ class Win32FlutterWindow : public Win32Window {
   // |Win32Window|
   void OnPointerUp(double x, double y) override;
 
+  // |Win32Window|
+  void OnPointerLeave() override;
+
   // |Win32Window|
   void OnChar(char32_t code_point) override;
 
@@ -105,6 +108,13 @@ class Win32FlutterWindow : public Win32Window {
   // Reports mouse release to Flutter engine.
   void SendPointerUp(double x, double y);
 
+  // Reports mouse left the window client area.
+  //
+  // Win32 api doesn't have "mouse enter" event. Therefore, there is no
+  // SendPointerEnter method. A mouse enter event is tracked then the "move"
+  // event is called.
+  void SendPointerLeave();
+
   // Reports a keyboard character to Flutter engine.
   void SendChar(char32_t code_point);
 

shell/platform/windows/win32_window.cc

@@ -98,6 +98,17 @@ LRESULT CALLBACK Win32Window::WndProc(HWND const window,
   return DefWindowProc(window, message, wparam, lparam);
 }
 
+void Win32Window::TrackMouseLeaveEvent(HWND hwnd) {
+  if (!tracking_mouse_leave_) {
+    TRACKMOUSEEVENT tme;
+    tme.cbSize = sizeof(tme);
+    tme.hwndTrack = hwnd;
+    tme.dwFlags = TME_LEAVE;
+    TrackMouseEvent(&tme);
+    tracking_mouse_leave_ = true;
+  }
+}
+
 LRESULT
 Win32Window::MessageHandler(HWND hwnd,
                             UINT const message,
@@ -120,7 +131,6 @@ Win32Window::MessageHandler(HWND hwnd,
         window->OnClose();
         return 0;
         break;
-
       case WM_SIZE:
         width = LOWORD(lparam);
         height = HIWORD(lparam);
@@ -133,12 +143,20 @@ Win32Window::MessageHandler(HWND hwnd,
         window->OnFontChange();
         break;
       case WM_MOUSEMOVE:
+        window->TrackMouseLeaveEvent(hwnd);
+
         xPos = GET_X_LPARAM(lparam);
         yPos = GET_Y_LPARAM(lparam);
-
         window->OnPointerMove(static_cast<double>(xPos),
                               static_cast<double>(yPos));
         break;
+      case WM_MOUSELEAVE:;
+        window->OnPointerLeave();
+        // Once the tracked event is received, the TrackMouseEvent function
+        // resets. Set to false to make sure it's called once mouse movement is
+        // detected again.
+        tracking_mouse_leave_ = false;
+        break;
       case WM_LBUTTONDOWN:
         xPos = GET_X_LPARAM(lparam);
         yPos = GET_Y_LPARAM(lparam);

shell/platform/windows/win32_window.h

@@ -88,6 +88,9 @@ class Win32Window {
   // down to up
   virtual void OnPointerUp(double x, double y) = 0;
 
+  // Called when the mouse leaves the window.
+  virtual void OnPointerLeave() = 0;
+
   // Called when character input occurs.
   virtual void OnChar(char32_t code_point) = 0;
 
@@ -110,6 +113,9 @@ class Win32Window {
   UINT GetCurrentHeight();
 
  private:
+  // Activates tracking for a "mouse leave" event.
+  void TrackMouseLeaveEvent(HWND hwnd);
+
   // Stores new width and height and calls |OnResize| to notify inheritors
   void HandleResize(UINT width, UINT height);
 
@@ -133,6 +139,9 @@ class Win32Window {
   // aspects of win32 High DPI handling across different OS versions.
   std::unique_ptr<Win32DpiHelper> dpi_helper_ =
       std::make_unique<Win32DpiHelper>();
+
+  // Set to true to be notified when the mouse leaves the window.
+  bool tracking_mouse_leave_ = false;
 };
 
 }  // namespace flutter