dnd: send DragEnter and DragMove on DnD start

This was a regression from Qt4 and also is the documented behavior.
In addition this patch fixes various issues with cursor shape updating
that were discovered along the way and that are necessary for testing
the new changes.

The code in QGuiApplicationPrivate::processDrag() also needed a fixup,
particularly the resetting of QGuiApplicationPrivate::currentDragWindow.
Without this fix we would get DragMove (the one that immediately follows
the DragEnter) only for the first DragEnter event. For example when dnd
starts on mouse press then for mouse click we would get:

<click> DragEnter->DragMove->DragLeave <click> DragEnter->DragLeave

but the expected is:

<click> DragEnter->DragMove->DragLeave <click> DragEnter->DragMove->DragLeave

Task-number: QTBUG-34331
Change-Id: I3cc96c87d1fd5d1342c7f6c9438802ab30076e9e
Reviewed-by: Shawn Rutledge <shawn.rutledge@qt.io>

7 files changed, 90 insertions, 65 deletions

diff --git a/src/gui/kernel/qguiapplication.cpp b/src/gui/kernel/qguiapplication.cpp
index 1ebdde3426..10defabd4f 100644
--- a/src/gui/kernel/qguiapplication.cpp
+++ b/src/gui/kernel/qguiapplication.cpp
@@ -143,6 +143,8 @@ Qt::ApplicationState QGuiApplicationPrivate::applicationState = Qt::ApplicationI
 
 bool QGuiApplicationPrivate::highDpiScalingUpdated = false;
 
+QPointer<QWindow> QGuiApplicationPrivate::currentDragWindow;
+
 QVector<QGuiApplicationPrivate::TabletPointData> QGuiApplicationPrivate::tabletDevicePoints;
 
 QPlatformIntegration *QGuiApplicationPrivate::platform_integration = 0;
@@ -668,6 +670,7 @@ QGuiApplication::~QGuiApplication()
     QGuiApplicationPrivate::currentMousePressWindow = QGuiApplicationPrivate::currentMouseWindow = nullptr;
     QGuiApplicationPrivate::applicationState = Qt::ApplicationInactive;
     QGuiApplicationPrivate::highDpiScalingUpdated = false;
+    QGuiApplicationPrivate::currentDragWindow = nullptr;
     QGuiApplicationPrivate::tabletDevicePoints.clear();
 #ifndef QT_NO_SESSIONMANAGER
     QGuiApplicationPrivate::is_fallback_session_management_enabled = true;
@@ -3092,7 +3095,6 @@ QPlatformDragQtResponse QGuiApplicationPrivate::processDrag(QWindow *w, const QM
 {
     updateMouseAndModifierButtonState(buttons, modifiers);
 
-    static QPointer<QWindow> currentDragWindow;
     static Qt::DropAction lastAcceptedDropAction = Qt::IgnoreAction;
     QPlatformDrag *platformDrag = platformIntegration()->drag();
     if (!platformDrag || (w && w->d_func()->blockedByModalWindow)) {
@@ -3101,8 +3103,7 @@ QPlatformDragQtResponse QGuiApplicationPrivate::processDrag(QWindow *w, const QM
     }
 
     if (!dropData) {
-        if (currentDragWindow.data() == w)
-            currentDragWindow = 0;
+        currentDragWindow = nullptr;
         QDragLeaveEvent e;
         QGuiApplication::sendEvent(w, &e);
         lastAcceptedDropAction = Qt::IgnoreAction;
@@ -3141,6 +3142,8 @@ QPlatformDropQtResponse QGuiApplicationPrivate::processDrop(QWindow *w, const QM
 {
     updateMouseAndModifierButtonState(buttons, modifiers);
 
+    currentDragWindow = nullptr;
+
     QDropEvent de(p, supportedActions, dropData, buttons, modifiers);
     QGuiApplication::sendEvent(w, &de);
 
diff --git a/src/gui/kernel/qguiapplication_p.h b/src/gui/kernel/qguiapplication_p.h
index b7ee9f86b5..145e764244 100644
--- a/src/gui/kernel/qguiapplication_p.h
+++ b/src/gui/kernel/qguiapplication_p.h
@@ -217,6 +217,7 @@ public:
     static QWindow *currentMousePressWindow;
     static Qt::ApplicationState applicationState;
     static bool highDpiScalingUpdated;
+    static QPointer<QWindow> currentDragWindow;
 
     struct TabletPointData {
         TabletPointData(qint64 devId = 0) : deviceId(devId), state(Qt::NoButton), target(nullptr) {}
diff --git a/src/gui/kernel/qplatformintegration.cpp b/src/gui/kernel/qplatformintegration.cpp
index dfb8f60915..6e285a8fa5 100644
--- a/src/gui/kernel/qplatformintegration.cpp
+++ b/src/gui/kernel/qplatformintegration.cpp
@@ -99,8 +99,8 @@ QPlatformClipboard *QPlatformIntegration::clipboard() const
 /*!
     Accessor for the platform integration's drag object.
 
-    Default implementation returns 0, implying no drag and drop support.
-
+    Default implementation returns QSimpleDrag. This class supports only drag
+    and drop operations within the same Qt application.
 */
 QPlatformDrag *QPlatformIntegration::drag() const
 {
diff --git a/src/gui/kernel/qsimpledrag.cpp b/src/gui/kernel/qsimpledrag.cpp
index 35896514b0..a84f873437 100644
--- a/src/gui/kernel/qsimpledrag.cpp
+++ b/src/gui/kernel/qsimpledrag.cpp
@@ -94,11 +94,7 @@ static QWindow* topLevelAt(const QPoint &pos)
     (within the Qt application or outside) accepts the drag and sets the state accordingly.
 */
 
-QBasicDrag::QBasicDrag() :
-    m_current_window(nullptr), m_restoreCursor(false), m_eventLoop(nullptr),
-    m_executed_drop_action(Qt::IgnoreAction), m_can_drop(false),
-    m_drag(nullptr), m_drag_icon_window(nullptr), m_useCompositing(true),
-    m_screen(nullptr)
+QBasicDrag::QBasicDrag()
 {
 }
 
@@ -181,9 +177,9 @@ bool QBasicDrag::eventFilter(QObject *o, QEvent *e)
             // make the event relative to the window where the drag started. (QTBUG-66103)
             const QMouseEvent *release = static_cast<QMouseEvent *>(e);
             const QWindow *releaseWindow = topLevelAt(release->globalPos());
-            qCDebug(lcDnd) << "mouse released over" << releaseWindow << "after drag from" << m_current_window << "globalPos" << release->globalPos();
+            qCDebug(lcDnd) << "mouse released over" << releaseWindow << "after drag from" << m_sourceWindow << "globalPos" << release->globalPos();
             if (!releaseWindow)
-                releaseWindow = m_current_window;
+                releaseWindow = m_sourceWindow;
             QPoint releaseWindowPos = (releaseWindow ? releaseWindow->mapFromGlobal(release->globalPos()) : release->globalPos());
             QMouseEvent *newRelease = new QMouseEvent(release->type(),
                 releaseWindowPos, releaseWindowPos, release->screenPos(),
@@ -206,18 +202,15 @@ Qt::DropAction QBasicDrag::drag(QDrag *o)
     m_drag = o;
     m_executed_drop_action = Qt::IgnoreAction;
     m_can_drop = false;
-    m_restoreCursor = true;
-#ifndef QT_NO_CURSOR
-    qApp->setOverrideCursor(Qt::DragCopyCursor);
-    updateCursor(m_executed_drop_action);
-#endif
+
     startDrag();
     m_eventLoop = new QEventLoop;
     m_eventLoop->exec();
     delete m_eventLoop;
-    m_eventLoop = 0;
-    m_drag = 0;
+    m_eventLoop = nullptr;
+    m_drag = nullptr;
     endDrag();
+
     return m_executed_drop_action;
 }
 
@@ -229,16 +222,6 @@ void QBasicDrag::cancelDrag()
     }
 }
 
-void QBasicDrag::restoreCursor()
-{
-    if (m_restoreCursor) {
-#ifndef QT_NO_CURSOR
-        QGuiApplication::restoreOverrideCursor();
-#endif
-        m_restoreCursor = false;
-    }
-}
-
 void QBasicDrag::startDrag()
 {
     QPoint pos;
@@ -320,25 +303,34 @@ void QBasicDrag::updateCursor(Qt::DropAction action)
         }
     }
 
-    QCursor *cursor = QGuiApplication::overrideCursor();
     QPixmap pixmap = m_drag->dragCursor(action);
-    if (!cursor) {
-        QGuiApplication::changeOverrideCursor((pixmap.isNull()) ? QCursor(cursorShape) : QCursor(pixmap));
+
+    if (!m_dndHasSetOverrideCursor) {
+        QCursor newCursor = !pixmap.isNull() ? QCursor(pixmap) : QCursor(cursorShape);
+        QGuiApplication::setOverrideCursor(newCursor);
+        m_dndHasSetOverrideCursor = true;
     } else {
+        QCursor *cursor = QGuiApplication::overrideCursor();
         if (!pixmap.isNull()) {
-            if ((cursor->pixmap().cacheKey() != pixmap.cacheKey())) {
+            if (cursor->pixmap().cacheKey() != pixmap.cacheKey())
                 QGuiApplication::changeOverrideCursor(QCursor(pixmap));
-            }
-        } else {
-            if (cursorShape != cursor->shape()) {
-                QGuiApplication::changeOverrideCursor(QCursor(cursorShape));
-            }
+        } else if (cursorShape != cursor->shape()) {
+            QGuiApplication::changeOverrideCursor(QCursor(cursorShape));
         }
     }
 #endif
     updateAction(action);
 }
 
+void QBasicDrag::restoreCursor()
+{
+#ifndef QT_NO_CURSOR
+    if (m_dndHasSetOverrideCursor) {
+        QGuiApplication::restoreOverrideCursor();
+        m_dndHasSetOverrideCursor = false;
+    }
+#endif
+}
 
 static inline QPoint fromNativeGlobalPixels(const QPoint &point)
 {
@@ -376,35 +368,38 @@ QSimpleDrag::QSimpleDrag()
 
 void QSimpleDrag::startDrag()
 {
+    setExecutedDropAction(Qt::IgnoreAction);
+
     QBasicDrag::startDrag();
     // Here we can be fairly sure that QGuiApplication::mouseButtons/keyboardModifiers() will
     // contain sensible values as startDrag() normally is called from mouse event handlers
     // by QDrag::exec(). A better API would be if we could pass something like "input device
     // pointer" to QDrag::exec(). My guess is that something like that might be required for
     // QTBUG-52430.
-    m_current_window = topLevelAt(QCursor::pos());
-    if (m_current_window) {
-        auto nativePixelPos = QHighDpi::toNativePixels(QCursor::pos(), m_current_window);
-        QPlatformDragQtResponse response = QWindowSystemInterface::handleDrag(
-                    m_current_window, drag()->mimeData(), nativePixelPos,
-                    drag()->supportedActions(), QGuiApplication::mouseButtons(),
-                    QGuiApplication::keyboardModifiers());
-        setCanDrop(response.isAccepted());
-        updateCursor(response.acceptedAction());
+    m_sourceWindow = topLevelAt(QCursor::pos());
+    m_windowUnderCursor = m_sourceWindow;
+    if (m_sourceWindow) {
+        auto nativePixelPos = QHighDpi::toNativePixels(QCursor::pos(), m_sourceWindow);
+        move(nativePixelPos, QGuiApplication::mouseButtons(), QGuiApplication::keyboardModifiers());
     } else {
         setCanDrop(false);
         updateCursor(Qt::IgnoreAction);
     }
-    setExecutedDropAction(Qt::IgnoreAction);
-    qCDebug(lcDnd) << "drag began from" << m_current_window<< "cursor pos" << QCursor::pos() << "can drop?" << canDrop();
+
+    qCDebug(lcDnd) << "drag began from" << m_sourceWindow << "cursor pos" << QCursor::pos() << "can drop?" << canDrop();
+}
+
+static void sendDragLeave(QWindow *window)
+{
+    QWindowSystemInterface::handleDrag(window, nullptr, QPoint(), Qt::IgnoreAction, 0, 0);
 }
 
 void QSimpleDrag::cancel()
 {
     QBasicDrag::cancel();
-    if (drag() && m_current_window) {
-        QWindowSystemInterface::handleDrag(m_current_window, nullptr, QPoint(), Qt::IgnoreAction, 0, 0);
-        m_current_window = nullptr;
+    if (drag() && m_sourceWindow) {
+        sendDragLeave(m_sourceWindow);
+        m_sourceWindow = nullptr;
     }
 }
 
@@ -414,16 +409,26 @@ void QSimpleDrag::move(const QPoint &nativeGlobalPos, Qt::MouseButtons buttons,
     QPoint globalPos = fromNativeGlobalPixels(nativeGlobalPos);
     moveShapedPixmapWindow(globalPos);
     QWindow *window = topLevelAt(globalPos);
-    if (!window)
-        return;
+
+    if (!window || window != m_windowUnderCursor) {
+        if (m_windowUnderCursor)
+            sendDragLeave(m_windowUnderCursor);
+        m_windowUnderCursor = window;
+        if (!window) {
+            // QSimpleDrag supports only in-process dnd, we can't drop anywhere else.
+            setCanDrop(false);
+            updateCursor(Qt::IgnoreAction);
+            return;
+        }
+    }
 
     const QPoint pos = nativeGlobalPos - window->handle()->geometry().topLeft();
     const QPlatformDragQtResponse qt_response = QWindowSystemInterface::handleDrag(
                 window, drag()->mimeData(), pos, drag()->supportedActions(),
                 buttons, modifiers);
 
-    updateCursor(qt_response.acceptedAction());
     setCanDrop(qt_response.isAccepted());
+    updateCursor(qt_response.acceptedAction());
 }
 
 void QSimpleDrag::drop(const QPoint &nativeGlobalPos, Qt::MouseButtons buttons,
diff --git a/src/gui/kernel/qsimpledrag_p.h b/src/gui/kernel/qsimpledrag_p.h
index ddd32bc5a5..f9e8a83a39 100644
--- a/src/gui/kernel/qsimpledrag_p.h
+++ b/src/gui/kernel/qsimpledrag_p.h
@@ -55,13 +55,14 @@
 #include <qpa/qplatformdrag.h>
 
 #include <QtCore/QObject>
+#include <QtCore/QPointer>
+#include <QtGui/QWindow>
 
 QT_REQUIRE_CONFIG(draganddrop);
 
 QT_BEGIN_NAMESPACE
 
 class QMouseEvent;
-class QWindow;
 class QEventLoop;
 class QDropData;
 class QShapedPixmapWindow;
@@ -106,7 +107,8 @@ protected:
     QDrag *drag() const { return m_drag; }
 
 protected:
-    QWindow *m_current_window;
+    QWindow *m_sourceWindow = nullptr;
+    QPointer<QWindow> m_windowUnderCursor = nullptr;
 
 private:
     void enableEventFilter();
@@ -114,14 +116,16 @@ private:
     void restoreCursor();
     void exitDndEventLoop();
 
-    bool m_restoreCursor;
-    QEventLoop *m_eventLoop;
-    Qt::DropAction m_executed_drop_action;
-    bool m_can_drop;
-    QDrag *m_drag;
-    QShapedPixmapWindow *m_drag_icon_window;
-    bool m_useCompositing;
-    QScreen *m_screen;
+#ifndef QT_NO_CURSOR
+    bool m_dndHasSetOverrideCursor = false;
+#endif
+    QEventLoop *m_eventLoop = nullptr;
+    Qt::DropAction m_executed_drop_action = Qt::IgnoreAction;
+    bool m_can_drop = false;
+    QDrag *m_drag = nullptr;
+    QShapedPixmapWindow *m_drag_icon_window = nullptr;
+    bool m_useCompositing = true;
+    QScreen *m_screen = nullptr;
 };
 
 class Q_GUI_EXPORT QSimpleDrag : public QBasicDrag
diff --git a/src/plugins/platforms/xcb/qxcbdrag.cpp b/src/plugins/platforms/xcb/qxcbdrag.cpp
index 999ae16897..20b01225cf 100644
--- a/src/plugins/platforms/xcb/qxcbdrag.cpp
+++ b/src/plugins/platforms/xcb/qxcbdrag.cpp
@@ -201,6 +201,9 @@ void QXcbDrag::startDrag()
     QBasicDrag::startDrag();
     if (connection()->mouseGrabber() == nullptr)
         shapedPixmapWindow()->setMouseGrabEnabled(true);
+
+    auto nativePixelPos = QHighDpi::toNativePixels(QCursor::pos(), initiatorWindow);
+    move(nativePixelPos, QGuiApplication::mouseButtons(), QGuiApplication::keyboardModifiers());
 }
 
 void QXcbDrag::endDrag()
diff --git a/src/plugins/platforms/xcb/qxcbintegration.cpp b/src/plugins/platforms/xcb/qxcbintegration.cpp
index b8a0e85465..49649eb816 100644
--- a/src/plugins/platforms/xcb/qxcbintegration.cpp
+++ b/src/plugins/platforms/xcb/qxcbintegration.cpp
@@ -379,8 +379,17 @@ QPlatformClipboard *QXcbIntegration::clipboard() const
 #endif
 
 #if QT_CONFIG(draganddrop)
+#include <private/qsimpledrag_p.h>
 QPlatformDrag *QXcbIntegration::drag() const
 {
+    static const bool useSimpleDrag = qEnvironmentVariableIsSet("QT_XCB_USE_SIMPLE_DRAG");
+    if (Q_UNLIKELY(useSimpleDrag)) { // This is useful for testing purposes
+        static QSimpleDrag *simpleDrag = nullptr;
+        if (!simpleDrag)
+            simpleDrag = new QSimpleDrag();
+        return simpleDrag;
+    }
+
     return m_connections.at(0)->drag();
 }
 #endif