widgetwindow: send DragMove for every DragEnter

... as specified in the documentation. This was a regression from Qt4
and can cause mouse cursor flickering durig dragging on e.g. custom
widget where some areas of the widget do not accept drag-and-drop.

Task-number: QTBUG-67155
Change-Id: Iaa6f9407181931ed8e3d6a8fec13fd59d3c8625d
Reviewed-by: Shawn Rutledge <shawn.rutledge@qt.io>

2 files changed, 67 insertions, 47 deletions

diff --git a/src/widgets/kernel/qwidgetwindow.cpp b/src/widgets/kernel/qwidgetwindow.cpp
index 5603c7cb20..1c73588b7c 100644
--- a/src/widgets/kernel/qwidgetwindow.cpp
+++ b/src/widgets/kernel/qwidgetwindow.cpp
@@ -311,8 +311,10 @@ bool QWidgetWindow::event(QEvent *event)
 
 #if QT_CONFIG(draganddrop)
     case QEvent::DragEnter:
+        handleDragEnterEvent(static_cast<QDragEnterEvent *>(event));
+        return true;
     case QEvent::DragMove:
-        handleDragEnterMoveEvent(static_cast<QDragMoveEvent *>(event));
+        handleDragMoveEvent(static_cast<QDragMoveEvent *>(event));
         return true;
     case QEvent::DragLeave:
         handleDragLeaveEvent(static_cast<QDragLeaveEvent *>(event));
@@ -849,62 +851,80 @@ void QWidgetWindow::handleWheelEvent(QWheelEvent *event)
 
 #if QT_CONFIG(draganddrop)
 
-void QWidgetWindow::handleDragEnterMoveEvent(QDragMoveEvent *event)
+static QWidget *findDnDTarget(QWidget *parent, const QPoint &pos)
 {
-     Q_ASSERT(event->type() ==QEvent::DragMove || !m_dragTarget);
     // Find a target widget under mouse that accepts drops (QTBUG-22987).
-    QWidget *widget = m_widget->childAt(event->pos());
+    QWidget *widget = parent->childAt(pos);
     if (!widget)
-        widget = m_widget;
+        widget = parent;
     for ( ; widget && !widget->isWindow() && !widget->acceptDrops(); widget = widget->parentWidget()) ;
     if (widget && !widget->acceptDrops())
-        widget = 0;
-    // Target widget unchanged: DragMove
-    if (widget && widget == m_dragTarget.data()) {
-        Q_ASSERT(event->type() == QEvent::DragMove);
-        const QPoint mapped = widget->mapFromGlobal(m_widget->mapToGlobal(event->pos()));
-        QDragMoveEvent translated(mapped, event->possibleActions(), event->mimeData(), event->mouseButtons(), event->keyboardModifiers());
-        translated.setDropAction(event->dropAction());
-        if (event->isAccepted()) { // Handling 'DragEnter' should suffice for the application.
-            translated.accept();
-            translated.setDropAction(event->dropAction());
-        }
-        QGuiApplication::forwardEvent(widget, &translated, event);
-        if (translated.isAccepted()) {
-            event->accept();
-        } else {
-            event->ignore();
-        }
-        event->setDropAction(translated.dropAction());
-        return;
-    }
-    // Target widget changed: Send DragLeave to previous, DragEnter to new if there is any
-    if (m_dragTarget.data()) {
-        QDragLeaveEvent le;
-        QGuiApplication::forwardEvent(m_dragTarget.data(), &le, event);
-        m_dragTarget = 0;
-    }
+        widget = nullptr;
+    return widget;
+}
+
+void QWidgetWindow::handleDragEnterEvent(QDragEnterEvent *event, QWidget *widget)
+{
+    Q_ASSERT(m_dragTarget == nullptr);
+    if (!widget)
+        widget = findDnDTarget(m_widget, event->pos());
     if (!widget) {
-         event->ignore();
-         return;
+        event->ignore();
+        return;
     }
     m_dragTarget = widget;
+
     const QPoint mapped = widget->mapFromGlobal(m_widget->mapToGlobal(event->pos()));
-    QDragEnterEvent translated(mapped, event->possibleActions(), event->mimeData(), event->mouseButtons(), event->keyboardModifiers());
-    QGuiApplication::forwardEvent(widget, &translated, event);
-    if (translated.isAccepted()) {
-        event->accept();
-    } else {
+    QDragEnterEvent translated(mapped, event->possibleActions(), event->mimeData(),
+                               event->mouseButtons(), event->keyboardModifiers());
+    translated.setDropAction(event->dropAction());
+    translated.setAccepted(event->isAccepted());
+    QGuiApplication::forwardEvent(m_dragTarget, &translated, event);
+    event->setAccepted(translated.isAccepted());
+    event->setDropAction(translated.dropAction());
+}
+
+void QWidgetWindow::handleDragMoveEvent(QDragMoveEvent *event)
+{
+    auto *widget = findDnDTarget(m_widget, event->pos());
+    if (!widget) {
         event->ignore();
+        if (m_dragTarget) { // Send DragLeave to previous
+            QDragLeaveEvent leaveEvent;
+            QGuiApplication::forwardEvent(m_dragTarget, &leaveEvent, event);
+            m_dragTarget = nullptr;
+        }
+    } else {
+        const QPoint mapped = widget->mapFromGlobal(m_widget->mapToGlobal(event->pos()));
+        QDragMoveEvent translated(mapped, event->possibleActions(), event->mimeData(),
+                                  event->mouseButtons(), event->keyboardModifiers());
+        translated.setDropAction(event->dropAction());
+        translated.setAccepted(event->isAccepted());
+
+        if (widget == m_dragTarget) { // Target widget unchanged: Send DragMove
+            QGuiApplication::forwardEvent(m_dragTarget, &translated, event);
+        } else {
+            if (m_dragTarget) { // Send DragLeave to previous
+                QDragLeaveEvent leaveEvent;
+                QGuiApplication::forwardEvent(m_dragTarget, &leaveEvent, event);
+                m_dragTarget = nullptr;
+            }
+            // Send DragEnter to new widget.
+            handleDragEnterEvent(static_cast<QDragEnterEvent*>(event), widget);
+            // The drag enter event is always immediately followed by a drag move event,
+            // see QDragEnterEvent documentation.
+            QGuiApplication::forwardEvent(m_dragTarget, &translated, event);
+        }
+        event->setAccepted(translated.isAccepted());
+        event->setDropAction(translated.dropAction());
     }
-    event->setDropAction(translated.dropAction());
 }
 
 void QWidgetWindow::handleDragLeaveEvent(QDragLeaveEvent *event)
 {
     if (m_dragTarget)
-        QGuiApplication::forwardEvent(m_dragTarget.data(), event);
-    m_dragTarget = 0;
+        QGuiApplication::forwardEvent(m_dragTarget, event);
+    m_dragTarget = nullptr;
 }
 
 void QWidgetWindow::handleDropEvent(QDropEvent *event)
@@ -914,13 +934,12 @@ void QWidgetWindow::handleDropEvent(QDropEvent *event)
         event->ignore();
         return;
     }
-    const QPoint mapped = m_dragTarget.data()->mapFromGlobal(m_widget->mapToGlobal(event->pos()));
+    const QPoint mapped = m_dragTarget->mapFromGlobal(m_widget->mapToGlobal(event->pos()));
     QDropEvent translated(mapped, event->possibleActions(), event->mimeData(), event->mouseButtons(), event->keyboardModifiers());
-    QGuiApplication::forwardEvent(m_dragTarget.data(), &translated, event);
-    if (translated.isAccepted())
-        event->accept();
+    QGuiApplication::forwardEvent(m_dragTarget, &translated, event);
+    event->setAccepted(translated.isAccepted());
     event->setDropAction(translated.dropAction());
-    m_dragTarget = 0;
+    m_dragTarget = nullptr;
 }
 
 #endif // QT_CONFIG(draganddrop)
diff --git a/src/widgets/kernel/qwidgetwindow_p.h b/src/widgets/kernel/qwidgetwindow_p.h
index ead099390e..0728135467 100644
--- a/src/widgets/kernel/qwidgetwindow_p.h
+++ b/src/widgets/kernel/qwidgetwindow_p.h
@@ -96,7 +96,8 @@ protected:
     void handleWheelEvent(QWheelEvent *);
 #endif
 #if QT_CONFIG(draganddrop)
-    void handleDragEnterMoveEvent(QDragMoveEvent *);
+    void handleDragEnterEvent(QDragEnterEvent *, QWidget *widget = nullptr);
+    void handleDragMoveEvent(QDragMoveEvent *);
     void handleDragLeaveEvent(QDragLeaveEvent *);
     void handleDropEvent(QDropEvent *);
 #endif