QQmlPropertyBinding: Do re-attach observer after handling undefined

As noted in the comments above, we need to restore the binding to its
original state after resetting the property. This includes re-adding any
observers.

Pick-to: 6.2 6.5 6.6
Fixes: QTBUG-114874
Change-Id: I2113a59f1bed59753c77ca3e0b00fed4c0487584
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>

2 files changed, 32 insertions, 0 deletions

diff --git a/tests/auto/qml/qqmlecmascript/data/restoreObserverAfterReset.qml b/tests/auto/qml/qqmlecmascript/data/restoreObserverAfterReset.qml
new file mode 100644
index 0000000000..f6edb07705
--- /dev/null
+++ b/tests/auto/qml/qqmlecmascript/data/restoreObserverAfterReset.qml
@@ -0,0 +1,20 @@
+import QtQuick
+
+Item {
+    height: undefined
+    implicitHeight: 30
+    property int steps: 0
+
+    Behavior on height {
+        NumberAnimation {
+            duration: 100
+        }
+    }
+
+    onHeightChanged: ++steps
+
+    Component.onCompleted: {
+        height = Qt.binding(() => implicitHeight);
+        implicitHeight = 60;
+    }
+}
diff --git a/tests/auto/qml/qqmlecmascript/tst_qqmlecmascript.cpp b/tests/auto/qml/qqmlecmascript/tst_qqmlecmascript.cpp
index 42fa04e759..53d068c855 100644
--- a/tests/auto/qml/qqmlecmascript/tst_qqmlecmascript.cpp
+++ b/tests/auto/qml/qqmlecmascript/tst_qqmlecmascript.cpp
@@ -378,6 +378,7 @@ private slots:
     void qpropertyBindingHandlesUndefinedCorrectly();
     void qpropertyBindingHandlesUndefinedWithoutResetCorrectly_data();
     void qpropertyBindingHandlesUndefinedWithoutResetCorrectly();
+    void qpropertyBindingRestoresObserverAfterReset();
     void hugeRegexpQuantifiers();
     void singletonTypeWrapperLookup();
     void getThisObject();
@@ -9429,6 +9430,17 @@ void tst_qqmlecmascript::qpropertyBindingHandlesUndefinedWithoutResetCorrectly()
     QCOMPARE(root->property("value2").toInt(), 2);
 }
 
+void tst_qqmlecmascript::qpropertyBindingRestoresObserverAfterReset()
+{
+    QQmlEngine engine;
+    QQmlComponent c(&engine, testFileUrl("restoreObserverAfterReset.qml"));
+    QVERIFY2(c.isReady(), qPrintable(c.errorString()));
+    QScopedPointer<QObject> o(c.create());
+    QVERIFY(!o.isNull());
+    QTRY_COMPARE(o->property("height").toDouble(), 60.0);
+    QVERIFY(o->property("steps").toInt() > 3);
+}
+
 void tst_qqmlecmascript::hugeRegexpQuantifiers()
 {
     QJSEngine engine;