Windows QPA: Fix QWheelEvent::buttons() after click on title bar

When the left or right mouse buttons are pressed over the window title
bar a WM_NCLBUTTONDOWN/WM_NCRBUTTONDOWN message is received. But when the
button is released, no corresponding UP message is received, but only
a WM_NCMOUSEMOVE or WM_MOUSEMOVE. This makes the internal mouse button
state stored in QGuiApplication get out of sync with the actual state,
resulting in an incorrect button state being used in QWheelEvent.
This patch detects the button release condition and generates the missing
release event.

Change-Id: I6dd9f8580bd6ba772522574f9a08298e49c43e61
Fixes: QTBUG-75678
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>

1 files changed, 37 insertions, 8 deletions

diff --git a/src/plugins/platforms/windows/qwindowsmousehandler.cpp b/src/plugins/platforms/windows/qwindowsmousehandler.cpp
index 737fd1d2a9..e33591c33d 100644
--- a/src/plugins/platforms/windows/qwindowsmousehandler.cpp
+++ b/src/plugins/platforms/windows/qwindowsmousehandler.cpp
@@ -157,6 +157,12 @@ QTouchDevice *QWindowsMouseHandler::ensureTouchDevice()
     return m_touchDevice;
 }
 
+void QWindowsMouseHandler::clearEvents()
+{
+    m_lastEventType = QEvent::None;
+    m_lastEventButton = Qt::NoButton;
+}
+
 Qt::MouseButtons QWindowsMouseHandler::queryMouseButtons()
 {
     Qt::MouseButtons result = nullptr;
@@ -287,8 +293,6 @@ bool QWindowsMouseHandler::translateMouseEvent(QWindow *window, HWND hwnd,
 
     Qt::MouseEventSource source = Qt::MouseEventNotSynthesized;
 
-    const MouseEvent mouseEvent = eventFromMsg(msg);
-
     // Check for events synthesized from touch. Lower byte is touch index, 0 means pen.
     static const bool passSynthesizedMouseEvents =
             !(QWindowsIntegration::instance()->options() & QWindowsIntegration::DontPassOsMouseEventsSynthesizedFromTouch);
@@ -305,13 +309,40 @@ bool QWindowsMouseHandler::translateMouseEvent(QWindow *window, HWND hwnd,
         }
     }
 
+    const Qt::KeyboardModifiers keyModifiers = QWindowsKeyMapper::queryKeyboardModifiers();
+    const MouseEvent mouseEvent = eventFromMsg(msg);
+    Qt::MouseButtons buttons;
+
+    if (mouseEvent.type >= QEvent::NonClientAreaMouseMove && mouseEvent.type <= QEvent::NonClientAreaMouseButtonDblClick)
+        buttons = queryMouseButtons();
+    else
+        buttons = keyStateToMouseButtons(msg.wParam);
+
+    // When the left/right mouse buttons are pressed over the window title bar
+    // WM_NCLBUTTONDOWN/WM_NCRBUTTONDOWN messages are received. But no UP
+    // messages are received on release, only WM_NCMOUSEMOVE/WM_MOUSEMOVE.
+    // We detect it and generate the missing release events here. (QTBUG-75678)
+    // The last event vars are cleared on QWindowsContext::handleExitSizeMove()
+    // to avoid generating duplicated release events.
+    if (m_lastEventType == QEvent::NonClientAreaMouseButtonPress
+            && (mouseEvent.type == QEvent::NonClientAreaMouseMove || mouseEvent.type == QEvent::MouseMove)
+            && (m_lastEventButton & buttons) == 0) {
+            if (mouseEvent.type == QEvent::NonClientAreaMouseMove) {
+                QWindowSystemInterface::handleFrameStrutMouseEvent(window, clientPosition, globalPosition, buttons, m_lastEventButton,
+                                                                   QEvent::NonClientAreaMouseButtonRelease, keyModifiers, source);
+            } else {
+                QWindowSystemInterface::handleMouseEvent(window, clientPosition, globalPosition, buttons, m_lastEventButton,
+                                                         QEvent::MouseButtonRelease, keyModifiers, source);
+            }
+    }
+    m_lastEventType = mouseEvent.type;
+    m_lastEventButton = mouseEvent.button;
+
     if (mouseEvent.type >= QEvent::NonClientAreaMouseMove && mouseEvent.type <= QEvent::NonClientAreaMouseButtonDblClick) {
-        const Qt::MouseButtons buttons = QWindowsMouseHandler::queryMouseButtons();
         QWindowSystemInterface::handleFrameStrutMouseEvent(window, clientPosition,
                                                            globalPosition, buttons,
                                                            mouseEvent.button, mouseEvent.type,
-                                                           QWindowsKeyMapper::queryKeyboardModifiers(),
-                                                           source);
+                                                           keyModifiers, source);
         return false; // Allow further event processing (dragging of windows).
     }
 
@@ -334,7 +365,6 @@ bool QWindowsMouseHandler::translateMouseEvent(QWindow *window, HWND hwnd,
     }
 
     QWindowsWindow *platformWindow = static_cast<QWindowsWindow *>(window->handle());
-    const Qt::MouseButtons buttons = keyStateToMouseButtons(int(msg.wParam));
 
     // If the window was recently resized via mouse doubleclick on the frame or title bar,
     // we don't get WM_LBUTTONDOWN or WM_LBUTTONDBLCLK for the second click,
@@ -461,8 +491,7 @@ bool QWindowsMouseHandler::translateMouseEvent(QWindow *window, HWND hwnd,
     if (!discardEvent && mouseEvent.type != QEvent::None) {
         QWindowSystemInterface::handleMouseEvent(window, winEventPosition, globalPosition, buttons,
                                                  mouseEvent.button, mouseEvent.type,
-                                                 QWindowsKeyMapper::queryKeyboardModifiers(),
-                                                 source);
+                                                 keyModifiers, source);
     }
     m_previousCaptureWindow = hasCapture ? window : nullptr;
     // QTBUG-48117, force synchronous handling for the extra buttons so that WM_APPCOMMAND