Don't let PointerHandler steal mouse grab from keepMouseGrab layer

As explained in the comment, the handler can override the keepMouseGrab
"veto" if the item is a parent (like a Flickable) that filters events,
but not in other cases.  The logic was wrong though, apparently.
Amends 090f404cf80da35734f712b02cc1543acecd5b62

Fixes: QTBUG-78258
Task-number: QTBUG-79163
Change-Id: I9a473ab3b23743f863cb0be13767fdbc29cd5e1c
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>
Reviewed-by: Shawn Rutledge <shawn.rutledge@qt.io>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
(cherry picked from commit b09ce7dcd8ecf24ef23da8197a64e3fced3fc894)

3 files changed, 105 insertions, 8 deletions

diff --git a/src/quick/handlers/qquickpointerhandler.cpp b/src/quick/handlers/qquickpointerhandler.cpp
index 92788fa0d7..b33b95eec1 100644
--- a/src/quick/handlers/qquickpointerhandler.cpp
+++ b/src/quick/handlers/qquickpointerhandler.cpp
@@ -351,10 +351,14 @@ bool QQuickPointerHandler::approveGrabTransition(QPointerEvent *event, const QEv
                         existingPhGrabber->metaObject()->className() == metaObject()->className())
                     allowed = true;
             } else if ((d->grabPermissions & CanTakeOverFromItems)) {
+                allowed = true;
                 QQuickItem * existingItemGrabber = qobject_cast<QQuickItem *>(event->exclusiveGrabber(point));
-                if (existingItemGrabber && !((existingItemGrabber->keepMouseGrab() && QQuickWindowPrivate::isMouseEvent(event)) ||
-                                             (existingItemGrabber->keepTouchGrab() && QQuickWindowPrivate::isTouchEvent(event)))) {
-                    allowed = true;
+                auto da = QQuickItemPrivate::get(parentItem())->deliveryAgentPrivate();
+                if (existingItemGrabber &&
+                        ((existingItemGrabber->keepMouseGrab() &&
+                          (QQuickWindowPrivate::isMouseEvent(event) || da->isDeliveringTouchAsMouse())) ||
+                         (existingItemGrabber->keepTouchGrab() && QQuickWindowPrivate::isTouchEvent(event)))) {
+                    allowed = false;
                     // If the handler wants to steal the exclusive grab from an Item, the Item can usually veto
                     // by having its keepMouseGrab flag set.  But an exception is if that Item is a parent that
                     // normally filters events (such as a Flickable): it needs to be possible for e.g. a
@@ -363,15 +367,20 @@ bool QQuickPointerHandler::approveGrabTransition(QPointerEvent *event, const QEv
                     // at first and then expects to be able to steal the grab later on.  It cannot respect
                     // Flickable's wishes in that case, because then it would never have a chance.
                     if (existingItemGrabber->keepMouseGrab() &&
-                            !(existingItemGrabber->filtersChildMouseEvents() && existingItemGrabber->isAncestorOf(parentItem()))) {
-                        auto da = QQuickItemPrivate::get(parentItem())->deliveryAgentPrivate();
+                            existingItemGrabber->filtersChildMouseEvents() && existingItemGrabber->isAncestorOf(parentItem())) {
                         Q_ASSERT(da);
                         if (da->isDeliveringTouchAsMouse() && point.id() == da->touchMouseId) {
-                            qCDebug(lcPointerHandlerGrab) << this << "wants to grab touchpoint" << point.id()
-                                << "but declines to steal grab from touch-mouse grabber with keepMouseGrab=true" << existingItemGrabber;
-                            allowed = false;
+                            qCDebug(lcPointerHandlerGrab) << this << "steals touchpoint" << point.id()
+                                << "despite parent touch-mouse grabber with keepMouseGrab=true" << existingItemGrabber;
+                            allowed = true;
                         }
                     }
+                    if (!allowed) {
+                        qCDebug(lcPointerHandlerGrab) << this << "wants to grab point" << point.id()
+                                << "but declines to steal from grabber" << existingItemGrabber
+                                << "with keepMouseGrab=" << existingItemGrabber->keepMouseGrab()
+                                << "keepTouchGrab=" << existingItemGrabber->keepTouchGrab();
+                    }
                 }
             }
         }
diff --git a/tests/auto/quick/pointerhandlers/qquickdraghandler/data/dragHandlerUnderModalLayer.qml b/tests/auto/quick/pointerhandlers/qquickdraghandler/data/dragHandlerUnderModalLayer.qml
new file mode 100644
index 0000000000..b24812c914
--- /dev/null
+++ b/tests/auto/quick/pointerhandlers/qquickdraghandler/data/dragHandlerUnderModalLayer.qml
@@ -0,0 +1,34 @@
+import QtQuick 2.15
+
+import Test 1.0
+
+Item {
+    width: 640
+    height: 480
+
+    Rectangle {
+        anchors.fill: parent
+        color: "grey"
+
+        Rectangle {
+            x: 200
+            y: 200
+            width: 100
+            height: 100
+            color: "orange"
+            DragHandler {
+                grabPermissions: DragHandler.CanTakeOverFromAnything // but not anything with keepMouseGrab!
+            }
+        }
+    }
+
+    ModalLayer {
+        anchors.fill: parent
+
+        Rectangle {
+            anchors.fill: parent
+            color: "red"
+            opacity: 0.4
+        }
+    }
+}
diff --git a/tests/auto/quick/pointerhandlers/qquickdraghandler/tst_qquickdraghandler.cpp b/tests/auto/quick/pointerhandlers/qquickdraghandler/tst_qquickdraghandler.cpp
index 4c9f015679..2cb26124ae 100644
--- a/tests/auto/quick/pointerhandlers/qquickdraghandler/tst_qquickdraghandler.cpp
+++ b/tests/auto/quick/pointerhandlers/qquickdraghandler/tst_qquickdraghandler.cpp
@@ -70,6 +70,7 @@ private slots:
     void touchPassiveGrabbers();
     void touchPinchAndMouseMove();
     void unsuitableEventDuringDrag();
+    void underModalLayer();
 
 private:
     void sendWheelEvent(QQuickView &window, QPoint pos, QPoint angleDelta, QPoint pixelDelta, Qt::KeyboardModifiers modifiers, Qt::ScrollPhase phase, bool inverted);
@@ -894,6 +895,59 @@ void tst_DragHandler::sendWheelEvent(QQuickView &window, QPoint pos, QPoint angl
     QQuickTouchUtils::flush(&window);
 }
 
+class ModalLayer : public QQuickItem {
+public:
+    explicit ModalLayer(QQuickItem* parent = nullptr) : QQuickItem(parent) {
+        this->setAcceptedMouseButtons(Qt::AllButtons);
+        this->setAcceptTouchEvents(true);
+        this->setKeepMouseGrab(true);
+        this->setKeepTouchGrab(true);
+    }
+
+    bool event(QEvent* event) override {
+        switch (event->type()) {
+        case QEvent::KeyPress:
+        case QEvent::MouseMove:
+        case QEvent::MouseButtonPress:
+        case QEvent::MouseButtonRelease:
+        case QEvent::MouseTrackingChange:
+        case QEvent::MouseButtonDblClick:
+        case QEvent::Wheel:
+        case QEvent::TouchBegin:
+        case QEvent::TouchUpdate:
+        case QEvent::TouchCancel:
+        case QEvent::TouchEnd: {
+            qCDebug(lcPointerTests) << "BLOCK!" << event->type();
+            return true;
+        }
+        default: break;
+        }
+        return QQuickItem::event(event);
+    }
+};
+
+void tst_DragHandler::underModalLayer() // QTBUG-78258
+{
+    qmlRegisterType<ModalLayer>("Test", 1, 0, "ModalLayer");
+
+    const int dragThreshold = QGuiApplication::styleHints()->startDragDistance();
+    QScopedPointer<QQuickView> windowPtr;
+    createView(windowPtr, "dragHandlerUnderModalLayer.qml");
+    QQuickView * window = windowPtr.data();
+    QPointer<QQuickDragHandler> dragHandler = window->rootObject()->findChild<QQuickDragHandler*>();
+    QVERIFY(dragHandler);
+
+    QPoint p1(250, 250);
+    QTest::mousePress(window, Qt::LeftButton, Qt::NoModifier, p1);
+    p1 += QPoint(dragThreshold, dragThreshold);
+    QTest::mouseMove(window, p1);
+    QVERIFY(!dragHandler->active());
+    p1 += QPoint(dragThreshold, dragThreshold);
+    QTest::mouseMove(window, p1);
+    QVERIFY(!dragHandler->active());
+    QTest::mouseRelease(window, Qt::LeftButton);
+}
+
 QTEST_MAIN(tst_DragHandler)
 
 #include "tst_qquickdraghandler.moc"