Windows QPA: Fix left button reported pressed after moving windows

When moving a window by dragging the title bar, no WM_NCLBUTTONUP is
received after WM_NCLBUTTONDOWN, WM_NCMOUSEMOVE (due to internal mouse
capture), which can leave the left mouse button 'pressed' in
QGuiApplication's state. Intercept WM_EXITSIZEMOVE to sync the buttons.

Complements 4589440891.

Change-Id: I94d18d1d4a4796dcecb1a9731809d05c7f9ddd65
Reviewed-by: Andre de la Rocha <andre.rocha@qt.io>
Reviewed-by: Oliver Wolff <oliver.wolff@qt.io>

src/plugins/platforms/windows/qwindowscontext.cpp

@@ -1093,6 +1093,7 @@ bool QWindowsContext::windowsProc(HWND hwnd, UINT message,
     case QtWindows::ExitSizeMoveEvent:
         platformWindow->clearFlag(QWindowsWindow::ResizeMoveActive);
         platformWindow->checkForScreenChanged();
+        QWindowsMouseHandler::handleExitSizeMove(platformWindow->window());
         return true;
     case QtWindows::ScrollEvent:
         return sessionManagerInteractionBlocked() || d->m_mouseHandler.translateScrollEvent(platformWindow->window(), hwnd, msg, result);

src/plugins/platforms/windows/qwindowsmousehandler.cpp

@@ -178,6 +178,30 @@ Qt::MouseButtons QWindowsMouseHandler::queryMouseButtons()
     return result;
 }
 
+void QWindowsMouseHandler::handleExitSizeMove(QWindow *window)
+{
+    // When moving a window by dragging the title bar, no WM_NCLBUTTONUP is
+    // received after WM_NCLBUTTONDOWN, WM_NCMOUSEMOVE (due to internal
+    // mouse capture), which can leave the left mouse button 'pressed'
+    // in QGuiApplication's state. Intercept WM_EXITSIZEMOVE to sync the buttons.
+    const Qt::MouseButtons currentButtons = QWindowsMouseHandler::queryMouseButtons();
+    const Qt::MouseButtons appButtons = QGuiApplication::mouseButtons();
+    if (currentButtons == appButtons)
+        return;
+    const Qt::KeyboardModifiers keyboardModifiers = QWindowsKeyMapper::queryKeyboardModifiers();
+    const QPoint globalPos = QWindowsCursor::mousePosition();
+    const QPoint localPos = window->handle()->mapFromGlobal(globalPos);
+    for (Qt::MouseButton button : {Qt::LeftButton, Qt::RightButton, Qt::MiddleButton}) {
+        if (appButtons.testFlag(button) && !currentButtons.testFlag(button)) {
+            QWindowSystemInterface::handleMouseEvent(window, localPos, globalPos,
+                                                     currentButtons, button,
+                                                     QEvent::NonClientAreaMouseButtonRelease,
+                                                     keyboardModifiers,
+                                                     Qt::MouseEventNotSynthesized);
+        }
+    }
+}
+
 static QPoint lastMouseMovePos;
 
 namespace {

src/plugins/platforms/windows/qwindowsmousehandler.h

@@ -80,6 +80,8 @@ public:
     QWindow *windowUnderMouse() const { return m_windowUnderMouse.data(); }
     void clearWindowUnderMouse() { m_windowUnderMouse = 0; }
 
+    static void handleExitSizeMove(QWindow *window);
+
 private:
     inline bool translateMouseWheelEvent(QWindow *window, HWND hwnd,
                                          MSG msg, LRESULT *result);