[Client] Guard dragged card ghost from painting or dropping deleted cards (#7688)

* [Client] Guard dragged card ghost from painting or dropping deleted cards

* [Game] Revert handling for cards destroyed mid-drag

Co-authored-by: sentry[bot] <39604003+sentry[bot]@users.noreply.github.com>

* [DeckView] Access pointer through QPointer in DeckViewCardDragItem

---------

Co-authored-by: sentry[bot] <39604003+sentry[bot]@users.noreply.github.com>
Co-authored-by: zeldazach <7460172+zeldazach@users.noreply.github.com>

cockatrice/src/game_graphics/board/abstract_card_drag_item.cpp

@@ -7,6 +7,7 @@
 #include <QBrush>
 #include <QColor>
 #include <QCursor>
+#include <QGraphicsScene>
 #include <QGraphicsSceneMouseEvent>
 #include <QPainter>
 #include <QPainterPath>
@@ -51,7 +52,20 @@ AbstractCardDragItem::AbstractCardDragItem(AbstractCardItem *_item,
                 update();
             });
 
-    connect(item, &QObject::destroyed, this, &AbstractCardDragItem::deleteLater);
+    // The source card can be destroyed mid-drag (e.g. moved by a server event). deleteLater() alone is not enough:
+    // a scene repaint can run before the deferred delete and paint the dead card. Detach from the scene immediately.
+    connect(_item, &QObject::destroyed, this, [this]() {
+        hide();
+        if (QGraphicsScene *sc = scene()) {
+            sc->removeItem(this);
+        }
+        deleteLater();
+    });
+}
+
+AbstractCardItem *AbstractCardDragItem::getItem() const
+{
+    return item.data();
 }
 
 QPainterPath AbstractCardDragItem::shape() const
@@ -65,6 +79,10 @@ QPainterPath AbstractCardDragItem::shape() const
 
 void AbstractCardDragItem::paint(QPainter *painter, const QStyleOptionGraphicsItem *option, QWidget *widget)
 {
+    if (item.isNull()) {
+        return;
+    }
+
     item->paint(painter, option, widget);
 
     // adds a mask to the card so it looks like the card hasnt been placed yet

cockatrice/src/game_graphics/board/abstract_card_drag_item.h

@@ -14,6 +14,7 @@
 #include <QList>
 #include <QObject>
 #include <QPointF>
+#include <QPointer>
 #include <QRectF>
 #include <qtmetamacros.h>
 
@@ -24,7 +25,8 @@ class AbstractCardDragItem : public QObject, public QGraphicsItem
     Q_OBJECT
     Q_INTERFACES(QGraphicsItem)
 protected:
-    AbstractCardItem *item;
+    /// Guarded pointer: the source card may be destroyed while this drag item still exists.
+    QPointer<AbstractCardItem> item;
     QPointF hotSpot;
     QList<AbstractCardDragItem *> childDrags;
 
@@ -44,10 +46,8 @@ public:
     }
     [[nodiscard]] QPainterPath shape() const override;
     void paint(QPainter *painter, const QStyleOptionGraphicsItem *option, QWidget *widget) override;
-    [[nodiscard]] AbstractCardItem *getItem() const
+    /// @return the source card, or nullptr if it has already been destroyed.
-    {
+    [[nodiscard]] AbstractCardItem *getItem() const;
-        return item;
-    }
     [[nodiscard]] QPointF getHotSpot() const
     {
         return hotSpot;

cockatrice/src/game_graphics/board/card_drag_item.cpp

@@ -130,8 +130,9 @@ void CardDragItem::mouseReleaseEvent(QGraphicsSceneMouseEvent *event)
     sc->removeItem(this);
 
     QList<CardDragItem *> dragItemList;
-    CardZoneLogic *startZone = static_cast<CardItem *>(item)->getZone();
+    CardZoneLogic *startZone = static_cast<CardItem *>(item.data())->getZone();
-    if (currentZone && !(static_cast<CardItem *>(item)->getAttachedTo() && (startZone == currentZone->getLogic()))) {
+    if (currentZone &&
+        !(static_cast<CardItem *>(item.data())->getAttachedTo() && (startZone == currentZone->getLogic()))) {
         if (!occupied) {
             dragItemList.append(this);
         }
@@ -139,7 +140,7 @@ void CardDragItem::mouseReleaseEvent(QGraphicsSceneMouseEvent *event)
         for (int i = 0; i < childDrags.size(); i++) {
             CardDragItem *c = static_cast<CardDragItem *>(childDrags[i]);
             if (!occupied &&
-                !(static_cast<CardItem *>(c->item)->getAttachedTo() && (startZone == currentZone->getLogic())) &&
+                !(static_cast<CardItem *>(c->item.data())->getAttachedTo() && (startZone == currentZone->getLogic())) &&
                 !c->occupied) {
                 dragItemList.append(c);
             }

cockatrice/src/game_graphics/deckview/deck_view.cpp

@@ -77,7 +77,7 @@ void DeckViewCardDragItem::updatePosition(const QPointF &cursorScenePos)
 
 void DeckViewCardDragItem::handleDrop(DeckViewCardContainer *target)
 {
-    auto *card = static_cast<DeckViewCard *>(item);
+    auto *card = static_cast<DeckViewCard *>(item.data());
     auto *start = static_cast<DeckViewCardContainer *>(item->parentItem());
     start->removeCard(card);
     target->addCard(card);