QQuickDrag: fix binding loop and re-entrancy for Automatic drag type

When Drag.dragType is Automatic and Drag.active is bound to e.g.
DragHandler.active, calling setActive(true) previously invoked
startDrag() immediately, which in turn called QDrag::exec(). That
exec() call enters a nested event loop, which is incompatible with
being called from within a QML binding or JS expression: the QML engine
is still mid-evaluation when exec() blocks, so when the pointer release
arrives in the nested loop, deactivates the DragHandler, and causes the
binding to re-trigger, the engine detects a binding loop.

Fix this by deferring the startDrag() call via Qt::QueuedConnection so
that the current JS frame unwinds completely before exec() runs from a
clean event-loop context. An 'if (d->active)' guard in the deferred
lambda handles the case where the drag is cancelled before the queued
call fires.

The executingNativeDrag flag is now set/cleared around exec() inside
startDrag() itself (covering both the Automatic binding path and direct
QML calls to Drag.startDrag()). It suppresses re-entrant setActive(false)
calls that arrive via DragHandler deactivation while exec() is blocking,
since startDrag() already handles all cleanup when exec() returns.

Fixes: QTBUG-144006
Change-Id: I9bf6a4e6f3edea7e12e187b5756ab483c462696a
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>
Reviewed-by: Konsta Alajärvi <konsta.alajarvi@qt.io>
Reviewed-by: Oliver Eftevaag <oliver.eftevaag@qt.io>
(cherry picked from commit ae6e67e4cf23af6dbceab5e62881864b3a38c867)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

2 files changed, 18 insertions, 4 deletions

diff --git a/src/quick/items/qquickdrag.cpp b/src/quick/items/qquickdrag.cpp
index c17027a2e4..316bbcddde 100644
--- a/src/quick/items/qquickdrag.cpp
+++ b/src/quick/items/qquickdrag.cpp
@@ -221,16 +221,26 @@ void QQuickDragAttached::setActive(bool active)
     if (d->active != active) {
         if (d->inEvent)
             qmlWarning(this) << "active cannot be changed from within a drag event handler";
-        else if (active) {
+        else if (d->executingNativeDrag) {
+            // QDrag::exec() is blocking in a nested event loop. Pointer release events
+            // processed there may deactivate the DragHandler and re-trigger this setter.
+            // Suppress: startDrag() already handles cleanup when exec() returns.
+        } else if (active) {
             if (d->dragType == QQuickDrag::Internal) {
                 d->start(d->supportedActions);
             } else {
                 d->active = true;
                 emit activeChanged();
                 if (d->dragType == QQuickDrag::Automatic) {
-                    // There are different semantics than start() since startDrag()
-                    // may be called after an internal drag is already started.
-                    d->startDrag(d->supportedActions);
+                    // QDrag::exec() enters a nested event loop; calling it directly
+                    // from a QML binding or JS expression would block the engine mid-
+                    // evaluation. Defer to a queued call so the current JS frame
+                    // unwinds completely before exec() runs from the event loop.
+                    QMetaObject::invokeMethod(this, [this]() {
+                        Q_D(QQuickDragAttached);
+                        if (d->active)
+                            d->startDrag(d->supportedActions);
+                    }, Qt::QueuedConnection);
                 }
             }
         }
@@ -849,7 +859,9 @@ Qt::DropAction QQuickDragAttachedPrivate::startDrag(Qt::DropActions supportedAct
     drag->setHotSpot(hotSpot.toPoint());
     emit q->dragStarted();
 
+    executingNativeDrag = true;
     Qt::DropAction dropAction = drag->exec(supportedActions);
+    executingNativeDrag = false;
 
     if (!QGuiApplicationPrivate::platformIntegration()->drag()->ownsDragObject())
         drag->deleteLater();
diff --git a/src/quick/items/qquickdrag_p_p.h b/src/quick/items/qquickdrag_p_p.h
index 06b6547951..23d1fa7be7 100644
--- a/src/quick/items/qquickdrag_p_p.h
+++ b/src/quick/items/qquickdrag_p_p.h
@@ -49,6 +49,7 @@ public:
         , active(false)
         , listening(false)
         , inEvent(false)
+        , executingNativeDrag(false)
         , dragRestarted(false)
         , itemMoved(false)
         , eventQueued(false)
@@ -83,6 +84,7 @@ public:
     bool active : 1;
     bool listening : 1;
     bool inEvent : 1;
+    bool executingNativeDrag : 1;
     bool dragRestarted : 1;
     bool itemMoved : 1;
     bool eventQueued : 1;