Fix native window drags losing mouse gestures on macOS and Windows

macOS: StartResizing dequeued the mouse-up in its tracking loop and
performWindowDragWithEvent: takes it too, so the view that got the
mouse-down never saw it. Flutter kept its button state down, reported the
next press as a move, and every second gesture was lost. Forward (or
synthesize) the mouse-up to the window.

Windows: StartDragging posted WM_NCLBUTTONDOWN without releasing the capture
held by the view under the press, so the system move loop never started.
Share one path with StartResizing: release capture, pass the cursor position,
and skip the hand-over when the primary button is already up, which would
otherwise glue the window to the cursor until the next click.

Found by tools/gui/flutter_window_drag_areas_test in the workspace.

Author: lijy91

src/platform/macos/window_macos.mm

@@ -705,10 +705,34 @@ static void NativeApiUpdateWindowClass(NSWindow* window, bool non_activating) {
   return [pimpl_->ns_window_ canBecomeKeyWindow];
 }
 
+// Ends the mouse gesture of the view that received the mouse-down. The native drag and resize
+// loops below take the rest of the gesture, including the mouse-up; a view that never sees it
+// keeps its button state down (Flutter then reports the next press as a move, and that whole
+// gesture is lost).
+static void NativeApiSendMouseUp(NSWindow* window, NSEvent* mouse_up) {
+  if (!mouse_up) {
+    mouse_up = [NSEvent mouseEventWithType:NSEventTypeLeftMouseUp
+                                  location:[window mouseLocationOutsideOfEventStream]
+                             modifierFlags:0
+                                 timestamp:[NSProcessInfo processInfo].systemUptime
+                              windowNumber:window.windowNumber
+                                   context:nil
+                               eventNumber:0
+                                clickCount:1
+                                  pressure:0];
+  }
+  [window sendEvent:mouse_up];
+}
+
 void Window::StartDragging() {
   NSWindow* window = pimpl_->ns_window_;
-  if (window.currentEvent) {
-    [window performWindowDragWithEvent:window.currentEvent];
+  NSEvent* event = window.currentEvent;
+  if (!event) {
+    return;
+  }
+  [window performWindowDragWithEvent:event];
+  if (event.type == NSEventTypeLeftMouseDown || event.type == NSEventTypeLeftMouseDragged) {
+    NativeApiSendMouseUp(window, nil);
   }
 }
 
@@ -745,6 +769,7 @@ static void NativeApiUpdateWindowClass(NSWindow* window, bool non_activating) {
                                            inMode:NSEventTrackingRunLoopMode
                                           dequeue:YES];
     if (!event || event.type == NSEventTypeLeftMouseUp) {
+      NativeApiSendMouseUp(window, event);
       break;
     }
 

src/platform/windows/window_windows.cpp

@@ -1153,10 +1153,27 @@ bool Window::IsFocusable() const {
   return (style & WS_DISABLED) == 0;
 }
 
+// Hands the mouse gesture in progress to the system frame, as if the press had landed on the
+// given non-client area (HTCAPTION moves, HTLEFT... resize).
+static void StartSystemFrameDrag(HWND hwnd, WPARAM hit_test) {
+  // Gesture recognizers can report a drag start after the button is already up (a plain
+  // click); the modal loop entered then would glue the window to the cursor until the next
+  // click.
+  const int primary = GetSystemMetrics(SM_SWAPBUTTON) ? VK_RBUTTON : VK_LBUTTON;
+  if ((GetAsyncKeyState(primary) & 0x8000) == 0) {
+    return;
+  }
+  // The caller is inside a mouse-down handler, which typically holds capture (Flutter's view
+  // does); release it, or the system frame never gets the drag.
+  ReleaseCapture();
+  POINT cursor;
+  GetCursorPos(&cursor);
+  PostMessage(hwnd, WM_NCLBUTTONDOWN, hit_test, MAKELPARAM(cursor.x, cursor.y));
+}
+
 void Window::StartDragging() {
   if (pimpl_->hwnd_) {
-    // Simulate dragging by sending WM_NCLBUTTONDOWN with HTCAPTION
-    PostMessage(pimpl_->hwnd_, WM_NCLBUTTONDOWN, HTCAPTION, 0);
+    StartSystemFrameDrag(pimpl_->hwnd_, HTCAPTION);
   }
 }
 
@@ -1192,12 +1209,7 @@ void Window::StartResizing(ResizeEdge edge) {
       hit_test = HTBOTTOMRIGHT;
       break;
   }
-  // The caller is inside a mouse-down handler, which typically holds capture;
-  // release it so the system frame can take over the drag.
-  ReleaseCapture();
-  POINT cursor;
-  GetCursorPos(&cursor);
-  PostMessage(pimpl_->hwnd_, WM_NCLBUTTONDOWN, hit_test, MAKELPARAM(cursor.x, cursor.y));
+  StartSystemFrameDrag(pimpl_->hwnd_, hit_test);
 }
 
 WindowId Window::GetId() const {