QWidget: don't re-polish when style did not change

Don't call style->unpolish()/polish() when the old and new style did not
change which might happen when setting a style sheet as this might
create an infinite loop and the style (re)sets some window attribute.
Bailing out early is not an option here as newStyle might be a nullptr so q->style() will fall back to the application default style.

Pick-to: 6.9
Fixes: QTBUG-133332
Change-Id: Ifa9ee4fdfa64b2768337e2d90b7bbaac5f3fcd70
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@qt.io>

src/widgets/kernel/qwidget.cpp

@@ -2676,7 +2676,7 @@ void QWidgetPrivate::setStyle_helper(QStyle *newStyle, bool propagate)
     extra->style = newStyle;
 
     // repolish
-    if (polished && q->windowType() != Qt::Desktop) {
+    if (polished && q->windowType() != Qt::Desktop && oldStyle != q->style()) {
         oldStyle->unpolish(q);
         q->style()->polish(q);
     }

tests/auto/widgets/kernel/qwidget/tst_qwidget.cpp

@@ -292,6 +292,7 @@ private slots:
     void setLocale();
     void propagateLocale();
     void deleteStyle();
+    void dontCrashOnSetStyle();
     void multipleToplevelFocusCheck();
     void setFocus();
 #ifndef QT_NO_CURSOR
@@ -6795,6 +6796,37 @@ void tst_QWidget::deleteStyle()
     QCoreApplication::processEvents();
 }
 
+class TestStyle : public QCommonStyle
+{
+    void polish(QWidget *w) override
+    {
+        w->setWindowFlag(Qt::NoDropShadowWindowHint, true);
+    }
+    void unpolish(QWidget *w) override
+    {
+        w->setWindowFlag(Qt::NoDropShadowWindowHint, false);
+    }
+};
+
+void tst_QWidget::dontCrashOnSetStyle()
+{
+    const auto oldStyleName = qApp->style()->name();
+    const auto resetStyleHelper = qScopeGuard([&] {
+        qApp->setStyleSheet({});
+        qApp->setStyle(QStyleFactory::create(oldStyleName));
+    });
+    {
+        qApp->setStyle(new TestStyle);
+        qApp->setStyleSheet("blub");
+        QComboBox w;
+        w.show();
+        QVERIFY(QTest::qWaitForWindowExposed(&w));
+        // this created an infinite loop / stack overflow inside setStyle_helper()
+        // directly call polish instead waiting for the polish event
+        qApp->style()->polish(&w);
+    }
+}
+
 class TopLevelFocusCheck: public QWidget
 {
     Q_OBJECT