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, 33 insertions, 4 deletions

diff --git a/src/plugins/platforms/windows/qwindowspointerhandler.cpp b/src/plugins/platforms/windows/qwindowspointerhandler.cpp
index 501b62e07a..fd3d711470 100644
--- a/src/plugins/platforms/windows/qwindowspointerhandler.cpp
+++ b/src/plugins/platforms/windows/qwindowspointerhandler.cpp
@@ -336,6 +336,12 @@ QTouchDevice *QWindowsPointerHandler::ensureTouchDevice()
     return m_touchDevice;
 }
 
+void QWindowsPointerHandler::clearEvents()
+{
+    m_lastEventType = QEvent::None;
+    m_lastEventButton = Qt::NoButton;
+}
+
 void QWindowsPointerHandler::handleCaptureRelease(QWindow *window,
                                                   QWindow *currentWindowUnderPointer,
                                                   HWND hwnd,
@@ -726,10 +732,35 @@ bool QWindowsPointerHandler::translateMouseEvent(QWindow *window,
     }
 
     const MouseEvent mouseEvent = eventFromMsg(msg);
+    Qt::MouseButtons mouseButtons;
+
+    if (mouseEvent.type >= QEvent::NonClientAreaMouseMove && mouseEvent.type <= QEvent::NonClientAreaMouseButtonDblClick)
+        mouseButtons = queryMouseButtons();
+    else
+        mouseButtons = mouseButtonsFromKeyState(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 & mouseButtons) == 0) {
+            if (mouseEvent.type == QEvent::NonClientAreaMouseMove) {
+                QWindowSystemInterface::handleFrameStrutMouseEvent(window, localPos, globalPos, mouseButtons, m_lastEventButton,
+                                                                   QEvent::NonClientAreaMouseButtonRelease, keyModifiers, source);
+            } else {
+                QWindowSystemInterface::handleMouseEvent(window, localPos, globalPos, mouseButtons, 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 nonclientButtons = queryMouseButtons();
-        QWindowSystemInterface::handleFrameStrutMouseEvent(window, localPos, globalPos, nonclientButtons,
+        QWindowSystemInterface::handleFrameStrutMouseEvent(window, localPos, globalPos, mouseButtons,
                                                            mouseEvent.button, mouseEvent.type, keyModifiers, source);
         return false; // Allow further event processing
     }
@@ -745,8 +776,6 @@ bool QWindowsPointerHandler::translateMouseEvent(QWindow *window,
         return true;
     }
 
-    const Qt::MouseButtons mouseButtons = mouseButtonsFromKeyState(msg.wParam);
-
     handleCaptureRelease(window, currentWindowUnderPointer, hwnd, mouseEvent.type, mouseButtons);
     handleEnterLeave(window, currentWindowUnderPointer, globalPos);