Resize offscreen window when QQuickWidget is resized

In a typical Qt Quick application, when a window is resized, the
contentItem of that window is resized with it, and then the root item.
QQuickOverlay in qtquickcontrols2 listens to size changes in the
contentItem (QQuickRootItem) via addItemChangeListener(), as a cheap
way (e.g. no signals) of knowing when to resize background dimming
effects. It resizes the dimmer item to the size of the window.

The first problem with QQuickWidget is that it only ever resizes the root item
when using the SizeRootObjectToView resize mode, and not the contentItem.

The second problem is that the root item is resized (via updateSize()) before
the window itself even has a size (which happens in
QQuickWidget::createFramebufferObject() via the call to
d->offscreenWindow->setGeometry()).

To demonstrate the second problem in detail, consider the following widget
hierarchy (written in everybody's favorite language: QML):

QMainWindow {
    QQuickWidget {
        QQuickWindow { // QQuickWidgetPrivate::offscreenWindow
            QQuickRootItem { // QQuickWindowPrivate::contentItem
                Page {} // QQuickWidgetPrivate::root
            }
        }
    }
}

The QMainWindow starts off as 200x200. When the window is resized,
QQuickWidget::resizeEvent() is called. The first thing it does is call
updateSize(), which in the case of SizeRootObjectToView, resizes the root item
to 300x300. This causes QQuickOverlayPrivate::itemGeometryChanged() to be
called, and the dimmers are resized to the size of the window, but the window
still has its 200x200 size, as it is only updated later, when
QQuickWidget::createFramebufferObject() is called.

This patch fixes these issues by ensuring that contentItem and the window
itself are resized along with the root item.

As to why such manual intervention is necessary: from what I can see, it is
because it's an "offscreen" window. This means that
QWindowPrivate::platformWindow is null, and setGeometry() takes a different
path that presumably results in no QResizeEvent being sent to the QQuickWindow.
As QQuickWindow relies on resizeEvent() being called to resize its contentItem,
the contentItem is never resized. With a typical Qt Quick application, all of
this works as expected.

Change-Id: I7401aa7a9b209096183416ab53014f67cceccbe4
Fixes: QTBUG-78323
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>
Reviewed-by: Laszlo Agocs <laszlo.agocs@qt.io>
Reviewed-by: Paul Olav Tvete <paul.tvete@qt.io>

1 files changed, 24 insertions, 9 deletions

diff --git a/src/quickwidgets/qquickwidget.cpp b/src/quickwidgets/qquickwidget.cpp
index 8ecfbf65dd..35cf06927a 100644
--- a/src/quickwidgets/qquickwidget.cpp
+++ b/src/quickwidgets/qquickwidget.cpp
@@ -107,6 +107,7 @@ void QQuickWidgetPrivate::init(QQmlEngine* e)
     renderControl = new QQuickWidgetRenderControl(q);
     offscreenWindow = new QQuickWindow(*new QQuickOffcreenWindowPrivate(),renderControl);
     offscreenWindow->setTitle(QString::fromLatin1("Offscreen"));
+    offscreenWindow->setObjectName(QString::fromLatin1("QQuickOffScreenWindow"));
     // Do not call create() on offscreenWindow.
 
     // Check if the Software Adaptation is being used
@@ -802,15 +803,29 @@ void QQuickWidgetPrivate::updateSize()
             q->updateGeometry();
         }
     } else if (resizeMode == QQuickWidget::SizeRootObjectToView) {
-        bool needToUpdateWidth = !qFuzzyCompare(q->width(), root->width());
-        bool needToUpdateHeight = !qFuzzyCompare(q->height(), root->height());
-
-        if (needToUpdateWidth && needToUpdateHeight)
-            root->setSize(QSizeF(q->width(), q->height()));
-        else if (needToUpdateWidth)
-            root->setWidth(q->width());
-        else if (needToUpdateHeight)
-            root->setHeight(q->height());
+        const bool needToUpdateWidth = !qFuzzyCompare(q->width(), root->width());
+        const bool needToUpdateHeight = !qFuzzyCompare(q->height(), root->height());
+
+        if (needToUpdateWidth && needToUpdateHeight) {
+            // Make sure that we have realistic sizing behavior by following
+            // what on-screen windows would do and resize everything, not just
+            // the root item. We do this because other types may be relying on
+            // us to behave correctly.
+            const QSizeF newSize(q->width(), q->height());
+            offscreenWindow->resize(newSize.toSize());
+            offscreenWindow->contentItem()->setSize(newSize);
+            root->setSize(newSize);
+        } else if (needToUpdateWidth) {
+            const int newWidth = q->width();
+            offscreenWindow->setWidth(newWidth);
+            offscreenWindow->contentItem()->setWidth(newWidth);
+            root->setWidth(newWidth);
+        } else if (needToUpdateHeight) {
+            const int newHeight = q->height();
+            offscreenWindow->setHeight(newHeight);
+            offscreenWindow->contentItem()->setHeight(newHeight);
+            root->setHeight(newHeight);
+        }
     }
 }