Use QQDeliveryAgent::sceneTransform, if set, in QQuickItem::mapFromGlobal

During dragging of a QEventPoint, Flickable computes the drag delta as
pos - mapFromGlobal(point.globalPressPosition())
We cannot use only the global position delta, because the Flickable
might be transformed in 2D (by setting rotation on it or in a parent, as
in tst_qquickflickable::clickAndDragWhenTransformed) or in 3D (by mapping
it onto a Model object). So we really need QQuickItem::mapFromGlobal()
to actually work regardless how many of these transformations are in
place. This is just the beginning: we have a lot of these mapFrom/To
functions; but it's enough for the Flickable in the quick3d/dynamictexture
example to work better. Without this fix, if you tried to drag a yellow
note on the door panel, at the very first drag ListView saw a large
delta and considered its drag threshold exceeded immediately, whereas
the DragHandler on the note saw a very small delta; so ListView grabbed
and DragHandler did not steal it: it relies on having "first dibs".
When the drag threshold is exceeded, Flickable merely plans to grab on
the next event rather than grabbing immediately, and therefore a child
has a chance to grab first. Therefore it's normally OK for DragHandler
to simply become the first exclusive grabber when the drag threshold is
exceeded, and not steal the grab from another item (although
grabPermissions can be changed to allow stealing if necessary).

However this means that we continue to enforce the drag threshold in
local (transformed) coordinates: if Flickable should wait until the
user drags 10 pixels, but it's scaled to half-size, it will start
dragging after only 5 pixels of movement, for example.

Task-number: QTBUG-92944
Change-Id: Id01cc833ca80850ca18a965adf47f435e43e20ed
Reviewed-by: Andy Nichols <andy.nichols@qt.io>
(cherry picked from commit f83efd3feb564cf8b929b9dfde6787853a561069)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

5 files changed, 44 insertions, 3 deletions

diff --git a/src/quick/handlers/qquickpointerhandler.cpp b/src/quick/handlers/qquickpointerhandler.cpp
index 11c29ee812..254f91cfe1 100644
--- a/src/quick/handlers/qquickpointerhandler.cpp
+++ b/src/quick/handlers/qquickpointerhandler.cpp
@@ -302,7 +302,8 @@ void QQuickPointerHandler::onGrabChanged(QQuickPointerHandler *grabber, QPointin
 */
 void QQuickPointerHandler::setPassiveGrab(QPointerEvent *event, const QEventPoint &point, bool grab)
 {
-    qCDebug(lcPointerHandlerGrab) << point << grab;
+    qCDebug(lcPointerHandlerGrab) << this << point << grab << "via"
+                                  << QQuickDeliveryAgentPrivate::currentOrItemDeliveryAgent(parentItem());
     if (grab) {
         event->addPassiveGrabber(point, this);
     } else {
diff --git a/src/quick/items/qquickitem.cpp b/src/quick/items/qquickitem.cpp
index 464076cbf8..05e50d8e75 100644
--- a/src/quick/items/qquickitem.cpp
+++ b/src/quick/items/qquickitem.cpp
@@ -8178,6 +8178,12 @@ QPointF QQuickItem::mapFromScene(const QPointF &point) const
     treated only as a hint. So, the resulting window position may differ from
     what is expected.
 
+    \note If this item is in a subscene, e.g. mapped onto a 3D
+    \l [QtQuick3D QML] {Model}{Model} object, the UV mapping is incorporated
+    into this transformation, so that it really goes from screen coordinates to
+    this item's coordinates, as long as \a point is actually within this item's bounds.
+    The other mapping functions do not yet work that way.
+
     \since 5.7
 
     \sa {Concepts - Visual Coordinates in Qt Quick}
@@ -8185,7 +8191,12 @@ QPointF QQuickItem::mapFromScene(const QPointF &point) const
 QPointF QQuickItem::mapFromGlobal(const QPointF &point) const
 {
     Q_D(const QQuickItem);
-    return mapFromScene(d->globalToWindowTransform().map(point));
+    QPointF scenePoint = d->globalToWindowTransform().map(point);
+    if (auto da = QQuickDeliveryAgentPrivate::currentOrItemDeliveryAgent(this)) {
+        if (auto sceneTransform = da->sceneTransform())
+            scenePoint = sceneTransform->map(scenePoint);
+    }
+    return mapFromScene(scenePoint);
 }
 
 /*!
diff --git a/src/quick/util/qquickdeliveryagent.cpp b/src/quick/util/qquickdeliveryagent.cpp
index a38ac410d4..3b045ed8e4 100644
--- a/src/quick/util/qquickdeliveryagent.cpp
+++ b/src/quick/util/qquickdeliveryagent.cpp
@@ -626,6 +626,21 @@ void QQuickDeliveryAgentPrivate::updateFocusItemTransform()
 }
 
 /*! \internal
+    If called during event delivery, returns the agent that is delivering the
+    event, without checking whether \a item is reachable from there.
+    Otherwise returns QQuickItemPrivate::deliveryAgent() (the delivery agent for
+    the narrowest subscene containing \a item), or \c null if \a item is \c null.
+*/
+QQuickDeliveryAgent *QQuickDeliveryAgentPrivate::currentOrItemDeliveryAgent(const QQuickItem *item)
+{
+    if (currentEventDeliveryAgent)
+        return currentEventDeliveryAgent;
+    if (item)
+        return QQuickItemPrivate::get(const_cast<QQuickItem *>(item))->deliveryAgent();
+    return nullptr;
+}
+
+/*! \internal
     QQuickDeliveryAgent delivers events to a tree of Qt Quick Items, beginning
     with the given root item, which is usually QQuickWindow::rootItem() but
     may alternatively be embedded into a Qt Quick 3D scene or something else.
@@ -650,8 +665,20 @@ QQuickItem *QQuickDeliveryAgent::rootItem() const
 }
 
 /*! \internal
+    Returns the object that was set in setSceneTransform(): a functor that
+    transforms from scene coordinates in the parent scene to scene coordinates
+    within this DA's subscene, or \c null if none was set.
+*/
+QQuickDeliveryAgent::Transform *QQuickDeliveryAgent::sceneTransform() const
+{
+    Q_D(const QQuickDeliveryAgent);
+    return d->sceneTransform;
+}
+
+/*! \internal
     QQuickDeliveryAgent takes ownership of the given \a transform, which
-    encapsulates the ability to transform viewport coordinates to rootItem coordinates.
+    encapsulates the ability to transform parent scene coordinates to rootItem
+    (subscene) coordinates.
 */
 void QQuickDeliveryAgent::setSceneTransform(QQuickDeliveryAgent::Transform *transform)
 {
diff --git a/src/quick/util/qquickdeliveryagent_p.h b/src/quick/util/qquickdeliveryagent_p.h
index 9b73d9b9b9..df8c97b368 100644
--- a/src/quick/util/qquickdeliveryagent_p.h
+++ b/src/quick/util/qquickdeliveryagent_p.h
@@ -81,6 +81,7 @@ public:
 
     QQuickItem *rootItem() const;
 
+    Transform *sceneTransform() const;
     void setSceneTransform(Transform *transform);
     bool event(QEvent *ev) override;
 
diff --git a/src/quick/util/qquickdeliveryagent_p_p.h b/src/quick/util/qquickdeliveryagent_p_p.h
index ad1eece36e..f931cfd70b 100644
--- a/src/quick/util/qquickdeliveryagent_p_p.h
+++ b/src/quick/util/qquickdeliveryagent_p_p.h
@@ -121,6 +121,7 @@ public:
     static bool subsceneAgentsExist;
     // QQuickDeliveryAgent::event() sets this to the one that's currently (trying to) handle the event
     static QQuickDeliveryAgent *currentEventDeliveryAgent;
+    static QQuickDeliveryAgent *currentOrItemDeliveryAgent(const QQuickItem *item);
 
     Qt::FocusReason lastFocusReason = Qt::OtherFocusReason;
     int pointerEventRecursionGuard = 0;