Windows QPA: Fix Drag&Drop from touchscreen/pen

The Drag&Drop functionality had stopped working with touchscreen/pen
after the WM_POINTER-based input handling was added. The Drag&Drop
functionality internally uses the DoDragDrop() WIN32 call which,
according to Microsoft docs, is not supported for invocation inside
handlers for touch/pen messages, and should be invoked in handlers
for mouse messages that are synthesized by the OS afterwards. The
result was that when DoDragDrop (which is a blocking function with
its own event loop) was called it would hang ignoring all touch/pen
messages until a mouse/touchpad message arrived. This change
implements a workaround for this issue by enqueuing Qt touch/pen
events that would be generated inside the pointer message handler,
and that could start a Drag&Drop operation, and only producing them
after the OS sends the associated mouse messages.

Task-number: QTBUG-70887
Change-Id: Id45e0ecc70358ba250de9b3268856781ed21c9dd
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>

src/plugins/platforms/windows/qwindowsdrag.cpp

@@ -652,6 +652,7 @@ QWindowsOleDropTarget::Drop(LPDATAOBJECT pDataObj, DWORD grfKeyState,
 */
 
 bool QWindowsDrag::m_canceled = false;
+bool QWindowsDrag::m_dragging = false;
 
 QWindowsDrag::QWindowsDrag() = default;
 
@@ -699,7 +700,10 @@ Qt::DropAction QWindowsDrag::drag(QDrag *drag)
     const DWORD allowedEffects = translateToWinDragEffects(possibleActions);
     qCDebug(lcQpaMime) << '>' << __FUNCTION__ << "possible Actions=0x"
         << hex << int(possibleActions) << "effects=0x" << allowedEffects << dec;
+    // Indicate message handlers we are in DoDragDrop() event loop.
+    QWindowsDrag::m_dragging = true;
     const HRESULT r = DoDragDrop(dropDataObject, windowDropSource, allowedEffects, &resultEffect);
+    QWindowsDrag::m_dragging = false;
     const DWORD  reportedPerformedEffect = dropDataObject->reportedPerformedEffect();
     if (r == DRAGDROP_S_DROP) {
         if (reportedPerformedEffect == DROPEFFECT_MOVE && resultEffect != DROPEFFECT_MOVE) {

src/plugins/platforms/windows/qwindowsdrag.h

@@ -92,6 +92,7 @@ public:
     static QWindowsDrag *instance();
     void cancelDrag() override { QWindowsDrag::m_canceled = true; }
     static bool isCanceled() { return QWindowsDrag::m_canceled; }
+    static bool isDragging() { return QWindowsDrag::m_dragging; }
 
     IDataObject *dropDataObject() const             { return m_dropDataObject; }
     void setDropDataObject(IDataObject *dataObject) { m_dropDataObject = dataObject; }
@@ -102,6 +103,7 @@ public:
 
 private:
     static bool m_canceled;
+    static bool m_dragging;
 
     QWindowsDropMimeData m_dropData;
     IDataObject *m_dropDataObject = nullptr;

src/plugins/platforms/windows/qwindowspointerhandler.cpp

@@ -50,6 +50,9 @@
 #include "qwindowswindow.h"
 #include "qwindowsintegration.h"
 #include "qwindowsscreen.h"
+#if QT_CONFIG(draganddrop)
+#  include "qwindowsdrag.h"
+#endif
 
 #include <qpa/qwindowsysteminterface.h>
 #include <QtGui/qguiapplication.h>
@@ -60,6 +63,7 @@
 #include <QtCore/qvarlengtharray.h>
 #include <QtCore/qloggingcategory.h>
 #include <QtCore/qoperatingsystemversion.h>
+#include <QtCore/qqueue.h>
 
 #include <algorithm>
 
@@ -75,6 +79,111 @@ enum {
     QT_PT_TOUCHPAD = 5, // MinGW is missing PT_TOUCHPAD
 };
 
+struct PointerTouchEventInfo {
+    QPointer<QWindow> window;
+    QList<QWindowSystemInterface::TouchPoint> points;
+    Qt::KeyboardModifiers modifiers;
+};
+
+struct PointerTabletEventInfo {
+    QPointer<QWindow> window;
+    QPointF local;
+    QPointF global;
+    int device;
+    int pointerType;
+    Qt::MouseButtons buttons;
+    qreal pressure;
+    int xTilt;
+    int yTilt;
+    qreal tangentialPressure;
+    qreal rotation;
+    int z;
+    qint64 uid;
+    Qt::KeyboardModifiers modifiers;
+};
+
+static QQueue<PointerTouchEventInfo> touchEventQueue;
+static QQueue<PointerTabletEventInfo> tabletEventQueue;
+
+static void enqueueTouchEvent(QWindow *window,
+                              const QList<QWindowSystemInterface::TouchPoint> &points,
+                              Qt::KeyboardModifiers modifiers)
+{
+    PointerTouchEventInfo eventInfo;
+    eventInfo.window = window;
+    eventInfo.points = points;
+    eventInfo.modifiers = modifiers;
+    touchEventQueue.enqueue(eventInfo);
+}
+
+static void enqueueTabletEvent(QWindow *window, const QPointF &local, const QPointF &global,
+                               int device, int pointerType, Qt::MouseButtons buttons, qreal pressure,
+                               int xTilt, int yTilt, qreal tangentialPressure, qreal rotation,
+                               int z, qint64 uid, Qt::KeyboardModifiers modifiers)
+{
+    PointerTabletEventInfo eventInfo;
+    eventInfo.window = window;
+    eventInfo.local = local;
+    eventInfo.global = global;
+    eventInfo.device = device;
+    eventInfo.pointerType = pointerType;
+    eventInfo.buttons = buttons;
+    eventInfo.pressure = pressure;
+    eventInfo.xTilt = xTilt;
+    eventInfo.yTilt = yTilt;
+    eventInfo.tangentialPressure = tangentialPressure;
+    eventInfo.rotation = rotation;
+    eventInfo.z = z;
+    eventInfo.uid = uid;
+    eventInfo.modifiers = modifiers;
+    tabletEventQueue.enqueue(eventInfo);
+}
+
+static void flushTouchEvents(QTouchDevice *touchDevice)
+{
+    while (!touchEventQueue.isEmpty()) {
+        PointerTouchEventInfo eventInfo = touchEventQueue.dequeue();
+        if (eventInfo.window) {
+            QWindowSystemInterface::handleTouchEvent(eventInfo.window,
+                                                     touchDevice,
+                                                     eventInfo.points,
+                                                     eventInfo.modifiers);
+        }
+    }
+}
+
+static void flushTabletEvents()
+{
+    while (!tabletEventQueue.isEmpty()) {
+        PointerTabletEventInfo eventInfo = tabletEventQueue.dequeue();
+        if (eventInfo.window) {
+            QWindowSystemInterface::handleTabletEvent(eventInfo.window,
+                                                      eventInfo.local,
+                                                      eventInfo.global,
+                                                      eventInfo.device,
+                                                      eventInfo.pointerType,
+                                                      eventInfo.buttons,
+                                                      eventInfo.pressure,
+                                                      eventInfo.xTilt,
+                                                      eventInfo.yTilt,
+                                                      eventInfo.tangentialPressure,
+                                                      eventInfo.rotation,
+                                                      eventInfo.z,
+                                                      eventInfo.uid,
+                                                      eventInfo.modifiers);
+        }
+    }
+}
+
+static bool draggingActive()
+{
+#if QT_CONFIG(draganddrop)
+    return QWindowsDrag::isDragging();
+#else
+    return false;
+#endif
+}
+
 bool QWindowsPointerHandler::translatePointerEvent(QWindow *window, HWND hwnd, QtWindows::WindowsEventType et, MSG msg, LRESULT *result)
 {
     *result = 0;
@@ -426,6 +535,9 @@ bool QWindowsPointerHandler::translateTouchEvent(QWindow *window, HWND hwnd,
     if (et & QtWindows::NonClientEventFlag)
         return false; // Let DefWindowProc() handle Non Client messages.
 
+    if (draggingActive())
+        return false; // Let DoDragDrop() loop handle it.
+
     if (count < 1)
         return false;
 
@@ -452,6 +564,8 @@ bool QWindowsPointerHandler::translateTouchEvent(QWindow *window, HWND hwnd,
 
     QList<QWindowSystemInterface::TouchPoint> touchPoints;
 
+    bool primaryPointer = false;
+
     if (QWindowsContext::verbose > 1)
         qCDebug(lcQpaEvents).noquote().nospace() << showbase
                 << __FUNCTION__
@@ -494,16 +608,23 @@ bool QWindowsPointerHandler::translateTouchEvent(QWindow *window, HWND hwnd,
             touchPoint.state = stationaryTouchPoint ? Qt::TouchPointStationary : Qt::TouchPointMoved;
             m_lastTouchPositions.insert(touchPoint.id, touchPoint.normalPosition);
         }
+        if (touchInfo[i].pointerInfo.pointerFlags & POINTER_FLAG_PRIMARY)
+            primaryPointer = true;
+
         touchPoints.append(touchPoint);
 
         // Avoid getting repeated messages for this frame if there are multiple pointerIds
         QWindowsContext::user32dll.skipPointerFrameMessages(touchInfo[i].pointerInfo.pointerId);
     }
-
+    if (primaryPointer) {
-    QWindowSystemInterface::handleTouchEvent(window, m_touchDevice, touchPoints,
+        // Postpone event delivery to avoid hanging inside DoDragDrop().
-                                             QWindowsKeyMapper::queryKeyboardModifiers());
+        // Only the primary pointer will generate mouse messages.
-
+        enqueueTouchEvent(window, touchPoints, QWindowsKeyMapper::queryKeyboardModifiers());
-    return true;
+    } else {
+        QWindowSystemInterface::handleTouchEvent(window, m_touchDevice, touchPoints,
+                                                 QWindowsKeyMapper::queryKeyboardModifiers());
+    }
+    return false; // Allow mouse messages to be generated.
 }
 
 bool QWindowsPointerHandler::translatePenEvent(QWindow *window, HWND hwnd, QtWindows::WindowsEventType et,
@@ -512,6 +633,9 @@ bool QWindowsPointerHandler::translatePenEvent(QWindow *window, HWND hwnd, QtWin
     if (et & QtWindows::NonClientEventFlag)
         return false; // Let DefWindowProc() handle Non Client messages.
 
+    if (draggingActive())
+        return false; // Let DoDragDrop() loop handle it.
+
     POINTER_PEN_INFO *penInfo = static_cast<POINTER_PEN_INFO *>(vPenInfo);
 
     RECT pRect, dRect;
@@ -592,20 +716,25 @@ bool QWindowsPointerHandler::translatePenEvent(QWindow *window, HWND hwnd, QtWin
         }
         const Qt::KeyboardModifiers keyModifiers = QWindowsKeyMapper::queryKeyboardModifiers();
 
-        QWindowSystemInterface::handleTabletEvent(target, localPos, hiResGlobalPos, device, type, mouseButtons,
+        // Postpone event delivery to avoid hanging inside DoDragDrop().
-                                                  pressure, xTilt, yTilt, tangentialPressure, rotation, z,
+        enqueueTabletEvent(target, localPos, hiResGlobalPos, device, type, mouseButtons,
-                                                  pointerId, keyModifiers);
+                           pressure, xTilt, yTilt, tangentialPressure, rotation, z,
-        break;
+                           pointerId, keyModifiers);
+        return false;  // Allow mouse messages to be generated.
     }
     }
     return true;
 }
 
-// SetCursorPos()/TrackMouseEvent() will generate old-style WM_MOUSE messages. Handle them here.
+// Process old-style mouse messages here.
 bool QWindowsPointerHandler::translateMouseEvent(QWindow *window, HWND hwnd, QtWindows::WindowsEventType et, MSG msg, LRESULT *result)
 {
     Q_UNUSED(et);
 
+    // Generate enqueued events.
+    flushTouchEvents(m_touchDevice);
+    flushTabletEvents();
+
     *result = 0;
     if (msg.message != WM_MOUSELEAVE && msg.message != WM_MOUSEMOVE)
         return false;