QWidget: deliver DragLeave events symmetrically

If a widget received a DragEnter event that it didn't accept, then the
UnderMouse widget attribute gets set. But the drag manager never got a
drag target, so the DragLeave event was never delivered, leaving the
UnderMouse attribute set incorrectly.

We always need to send DragLeave events to the receiver, even if the
DragEnter or DragMove was not accepted. Otherwise we are not in balance,
and the UnderMouse attribute will remain set.

This is a change of behavior and a very old bug, so only fixing this in
unreleased branches. Test case added to verify that explicitly generated
drag events result in the correct enter/leave events.

[ChangeLog][QtWidgets][QWidget] DragLeave events are now always sent to
the widget the mouse is leaving, even if it didn't accept the DragEnter
event.

Fixes: QTBUG-50403
Pick-to: 6.7
Change-Id: I5eae49da000fb4fea81f1767f0e73a06a6b78975
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>
Reviewed-by: Christian Ehrlicher <ch.ehrlicher@gmx.de>
Reviewed-by: Axel Spoerl <axel.spoerl@qt.io>

2 files changed, 106 insertions, 2 deletions

diff --git a/src/widgets/kernel/qapplication.cpp b/src/widgets/kernel/qapplication.cpp
index 163fb7beff..a17c6934fa 100644
--- a/src/widgets/kernel/qapplication.cpp
+++ b/src/widgets/kernel/qapplication.cpp
@@ -3039,8 +3039,16 @@ bool QApplication::notify(QObject *receiver, QEvent *e)
 #endif
                 w = qobject_cast<QWidget *>(QDragManager::self()->currentTarget());
 
-            if (!w)
-                break;
+            if (!w) {
+                // The widget that received DragEnter didn't accept the event, so we have no
+                // current drag target in the QDragManager. But DragLeave still needs to be
+                // dispatched so that enter/leave events are in balance (and so that UnderMouse
+                // gets cleared).
+                if (e->type() == QEvent::DragLeave)
+                    w = static_cast<QWidget *>(receiver);
+                else
+                    break;
+            }
             if (e->type() == QEvent::DragMove || e->type() == QEvent::Drop) {
                 QDropEvent *dragEvent = static_cast<QDropEvent *>(e);
                 QWidget *origReceiver = static_cast<QWidget *>(receiver);
diff --git a/tests/auto/widgets/kernel/qwidget/tst_qwidget.cpp b/tests/auto/widgets/kernel/qwidget/tst_qwidget.cpp
index 386c7c5503..a5e3201f01 100644
--- a/tests/auto/widgets/kernel/qwidget/tst_qwidget.cpp
+++ b/tests/auto/widgets/kernel/qwidget/tst_qwidget.cpp
@@ -13,6 +13,7 @@
 #include <qlineedit.h>
 #include <qlistview.h>
 #include <qmessagebox.h>
+#include <qmimedata.h>
 #include <qpainter.h>
 #include <qpoint.h>
 #include <qpushbutton.h>
@@ -37,6 +38,7 @@
 #include <QtGui/qbackingstore.h>
 #include <QtGui/qguiapplication.h>
 #include <QtGui/qpa/qplatformwindow.h>
+#include <QtGui/qpa/qplatformdrag.h>
 #include <QtGui/qscreen.h>
 #include <qmenubar.h>
 #include <qcompleter.h>
@@ -435,6 +437,8 @@ private slots:
 
     void setVisibleDuringDestruction();
 
+    void dragEnterLeaveSymmetry();
+
 private:
     const QString m_platform;
     QSize m_testWidgetSize;
@@ -13441,5 +13445,97 @@ void tst_QWidget::setVisibleDuringDestruction()
     QTRY_COMPARE(signalSpy.count(), 4);
 }
 
+/*!
+    Verify that we deliver DragEnter/Leave events symmetrically, even if the
+    widget entered didn't accept the DragEnter event.
+*/
+void tst_QWidget::dragEnterLeaveSymmetry()
+{
+    QWidget widget;
+    widget.setAcceptDrops(true);
+    QLineEdit lineEdit;
+    QLabel label("Hello world");
+    label.setAcceptDrops(true);
+
+    struct EventFilter : QObject
+    {
+        bool eventFilter(QObject *receiver, QEvent *event) override
+        {
+            switch (event->type()) {
+            case QEvent::DragEnter:
+            case QEvent::DragLeave:
+                receivers[event->type()] << receiver;
+                break;
+
+            default:
+                break;
+            }
+
+            return false;
+        }
+
+        QMap<QEvent::Type, QList<QObject *>> receivers;
+
+        void clear() { receivers.clear(); }
+        bool hasEntered(QWidget *widget) const
+        {
+            return receivers.value(QEvent::DragEnter).contains(widget);
+        }
+        bool hasLeft(QWidget *widget) const
+        {
+            return receivers.value(QEvent::DragLeave).contains(widget);
+        }
+    } filter;
+
+    widget.installEventFilter(&filter);
+    lineEdit.installEventFilter(&filter);
+    label.installEventFilter(&filter);
+
+    QVBoxLayout vbox;
+    vbox.setContentsMargins(10, 10, 10, 10);
+    vbox.addWidget(&lineEdit);
+    vbox.addWidget(&label);
+    widget.setLayout(&vbox);
+
+    widget.show();
+    QVERIFY(QTest::qWaitForWindowExposed(&widget));
+
+    QMimeData data;
+    data.setColorData(QVariant::fromValue(Qt::red));
+    QWindowSystemInterface::handleDrag(widget.windowHandle(), &data, QPoint(1, 1),
+                                       Qt::ActionMask, Qt::LeftButton, {});
+    QVERIFY(filter.hasEntered(&widget));
+    QVERIFY(!filter.hasEntered(&lineEdit));
+    QVERIFY(!filter.hasEntered(&label));
+    QVERIFY(widget.underMouse());
+    QVERIFY(!lineEdit.underMouse());
+    filter.clear();
+
+    QWindowSystemInterface::handleDrag(widget.windowHandle(), &data, lineEdit.geometry().center(),
+                                       Qt::ActionMask, Qt::LeftButton, {});
+    // DragEnter propagates as the lineEdit doesn't want it, so the widget
+    // sees both a Leave and an Enter event
+    QVERIFY(filter.hasLeft(&widget));
+    QVERIFY(filter.hasEntered(&widget));
+    QVERIFY(filter.hasEntered(&widget));
+    // both have the UnderMouse attribute set
+    QVERIFY(lineEdit.underMouse());
+    QVERIFY(widget.underMouse());
+
+    // The lineEdit didn't accept the DragEnter, but it should still has to
+    // get the DragLeave so that UnderMouse is cleared; the widget gets both
+    // Leave and Enter through propagation.
+    QWindowSystemInterface::handleDrag(widget.windowHandle(), &data, label.geometry().center(),
+                                       Qt::ActionMask, Qt::LeftButton, {});
+    QVERIFY(filter.hasLeft(&lineEdit));
+    QVERIFY(filter.hasLeft(&widget));
+    QVERIFY(filter.hasEntered(&label));
+    QVERIFY(filter.hasEntered(&widget));
+
+    QVERIFY(!lineEdit.underMouse());
+    QVERIFY(label.underMouse());
+    QVERIFY(widget.underMouse());
+}
+
 QTEST_MAIN(tst_QWidget)
 #include "tst_qwidget.moc"