Handle redirected rendering better in styles

Unlike in the QWidget-based desktop world, Qt Quick scenes can
be rendered in a variety of ways, some completely offscreen
wthout any native windows on screen, whereas some (most notably,
QQuickWidget) work offscreen but in association with an on-screen
window that is not the QQuickWindow. Therefore, every time a
QQuickWindow is accessed, typically from QQuickStyleItem, it needs
to be considered if further resolution is needed.

For devicePixelRatio, there is a handy helper available in form of
QQuickWindow::effectiveDevicePixelRatio(). This picks up the dpr
from either the QQuickWindow or the QQuickWidget's associated
top-level QWidget window (or whatever window a custom
QQuickRenderControl implementation reports).

Elsewhere, where we need a QWindow in order to do native window
things, QQuickRenderControl::renderWindowFor() must be called to see
if there is another QWindow we should be using in place of the
QQuickWindow.

Fixes: QTBUG-95937
Change-Id: I0690915d995ebb5f5cc0c48f565dfaf978e849ea
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Mitch Curtis <mitch.curtis@qt.io>
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@qt.io>
Reviewed-by: Samuel Ghinet <samuel.ghinet@qt.io>
(cherry picked from commit 6ee6b49bce210d7e86f68009a332360051f0e8fc)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

2 files changed, 54 insertions, 0 deletions

diff --git a/tests/auto/quickwidgets/qquickwidget/data/controls.qml b/tests/auto/quickwidgets/qquickwidget/data/controls.qml
new file mode 100644
index 0000000000..3e415f33da
--- /dev/null
+++ b/tests/auto/quickwidgets/qquickwidget/data/controls.qml
@@ -0,0 +1,23 @@
+import QtQuick
+import QtQuick.Controls
+import QtQuick.Layouts
+
+Rectangle {
+    id: root
+    ColumnLayout {
+        anchors.fill: parent
+        ComboBox {
+        }
+        Button {
+            text: "Button"
+        }
+        CheckBox {
+            text: "CheckBox"
+        }
+        RadioButton {
+            text: "RadioButton"
+        }
+        TextField {
+        }
+    }
+}
diff --git a/tests/auto/quickwidgets/qquickwidget/tst_qquickwidget.cpp b/tests/auto/quickwidgets/qquickwidget/tst_qquickwidget.cpp
index 6ae0dc12ac..c902cf56b3 100644
--- a/tests/auto/quickwidgets/qquickwidget/tst_qquickwidget.cpp
+++ b/tests/auto/quickwidgets/qquickwidget/tst_qquickwidget.cpp
@@ -144,6 +144,7 @@ private slots:
     void synthMouseFromTouch();
     void tabKey();
     void resizeOverlay();
+    void controls();
 
 private:
     QPointingDevice *device = QTest::createTouchDevice();
@@ -735,6 +736,36 @@ void tst_qquickwidget::resizeOverlay()
     QCOMPARE(overlay->height(), rootItem->height());
 }
 
+void tst_qquickwidget::controls()
+{
+    // Smoke test for having some basic Quick Controls in a scene in a QQuickWidget.
+    QWidget widget;
+    QVBoxLayout *contentVerticalLayout = new QVBoxLayout(&widget);
+    QQuickWidget *quickWidget = new QQuickWidget(testFileUrl("controls.qml"), &widget);
+    QCOMPARE(quickWidget->status(), QQuickWidget::Ready);
+    quickWidget->setResizeMode(QQuickWidget::SizeRootObjectToView);
+    contentVerticalLayout->addWidget(quickWidget);
+
+    widget.resize(400, 400);
+    widget.show();
+    QVERIFY(QTest::qWaitForWindowExposed(&widget));
+
+    QQuickItem *rootItem = qobject_cast<QQuickItem *>(quickWidget->rootObject());
+    QVERIFY(rootItem);
+    QCOMPARE(rootItem->size(), quickWidget->size());
+    QSize oldSize = quickWidget->size();
+
+    // Verify that QTBUG-95937 no longer occurs. (on Windows with the default
+    // native windows style this used to assert in debug builds)
+    widget.resize(300, 300);
+    QTRY_VERIFY(quickWidget->width() < oldSize.width());
+    QTRY_COMPARE(rootItem->size(), quickWidget->size());
+
+    widget.hide();
+    widget.show();
+    QVERIFY(QTest::qWaitForWindowExposed(&widget));
+}
+
 QTEST_MAIN(tst_qquickwidget)
 
 #include "tst_qquickwidget.moc"