TapHandler: don't (dis)connect signals to a null window6.7

This is unexpected, but we're told that it happens somehow.

Pick-to: 6.6 6.5
Fixes: QTBUG-108696
Change-Id: I738099f43003056f7a24c09b1dc36024a81cbbb1
Reviewed-by: Vladimir Belyavsky <belyavskyv@gmail.com>
Reviewed-by: Santhosh Kumar <santhosh.kumar.selvaraj@qt.io>
(cherry picked from commit 821f8f1f6c74b562bd984a12c959bc137dd2c11f)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

2 files changed, 18 insertions, 5 deletions

diff --git a/src/quick/handlers/qquicktaphandler.cpp b/src/quick/handlers/qquicktaphandler.cpp
index f662bcbc2a..accf307382 100644
--- a/src/quick/handlers/qquicktaphandler.cpp
+++ b/src/quick/handlers/qquicktaphandler.cpp
@@ -461,13 +461,25 @@ void QQuickTapHandler::onGrabChanged(QQuickPointerHandler *grabber, QPointingDev
 
 void QQuickTapHandler::connectPreRenderSignal(bool conn)
 {
+    // disconnect pre-existing connection, if any
+    disconnect(m_preRenderSignalConnection);
+
     auto par = parentItem();
-    if (!par)
+    if (!par || !par->window())
         return;
-    if (conn)
-        connect(par->window(), &QQuickWindow::beforeSynchronizing, this, &QQuickTapHandler::updateTimeHeld);
-    else
-        disconnect(par->window(), &QQuickWindow::beforeSynchronizing, this, &QQuickTapHandler::updateTimeHeld);
+
+    /*
+        Note: beforeSynchronizing is emitted from the SG thread, and the
+        timeHeldChanged signal can be used to do arbitrary things in user QML.
+
+        But the docs say the GUI thread is blockd, and "Therefore, it is safe
+        to access GUI thread thread data in a slot or lambda that is connected
+        with Qt::DirectConnection." We use the default AutoConnection just in case.
+    */
+    if (conn) {
+        m_preRenderSignalConnection = connect(par->window(), &QQuickWindow::beforeSynchronizing,
+                                              this, &QQuickTapHandler::updateTimeHeld);
+    }
 }
 
 void QQuickTapHandler::updateTimeHeld()
diff --git a/src/quick/handlers/qquicktaphandler_p.h b/src/quick/handlers/qquicktaphandler_p.h
index 7b78690572..c1e7d4bd50 100644
--- a/src/quick/handlers/qquicktaphandler_p.h
+++ b/src/quick/handlers/qquicktaphandler_p.h
@@ -103,6 +103,7 @@ private:
     QBasicTimer m_longPressTimer;
     QBasicTimer m_doubleTapTimer;
     QEventPoint m_singleTapReleasedPoint;
+    QMetaObject::Connection m_preRenderSignalConnection;
     Qt::MouseButton m_singleTapReleasedButton;
     int m_tapCount = 0;
     int m_longPressThreshold = -1;