1 files changed, 91 insertions, 39 deletions

diff --git a/src/widgets/kernel/qwidget.cpp b/src/widgets/kernel/qwidget.cpp
index ec7b5ce69a..30daee1b79 100644
--- a/src/widgets/kernel/qwidget.cpp
+++ b/src/widgets/kernel/qwidget.cpp
@@ -83,6 +83,7 @@ using namespace Qt::StringLiterals;
 
 Q_LOGGING_CATEGORY(lcWidgetPainting, "qt.widgets.painting", QtWarningMsg);
 Q_LOGGING_CATEGORY(lcWidgetShowHide, "qt.widgets.showhide", QtWarningMsg);
+Q_LOGGING_CATEGORY(lcWidgetWindow, "qt.widgets.window", QtWarningMsg);
 
 #ifndef QT_NO_DEBUG_STREAM
 namespace {
@@ -10953,57 +10954,61 @@ void QWidgetPrivate::setParent_sys(QWidget *newparent, Qt::WindowFlags f)
 
     setWinId(0);
 
-    if (parent != newparent) {
-        QObjectPrivate::setParent_helper(newparent); //### why does this have to be done in the _sys function???
-        if (q->windowHandle()) {
-            q->windowHandle()->setFlags(f);
-            QWidget *parentWithWindow =
-                newparent ? (newparent->windowHandle() ? newparent : newparent->nativeParentWidget()) : nullptr;
-            if (parentWithWindow) {
-                QWidget *topLevel = parentWithWindow->window();
-                if ((f & Qt::Window) && topLevel && topLevel->windowHandle()) {
-                    q->windowHandle()->setTransientParent(topLevel->windowHandle());
-                    q->windowHandle()->setParent(nullptr);
-                } else {
-                    q->windowHandle()->setTransientParent(nullptr);
-                    q->windowHandle()->setParent(parentWithWindow->windowHandle());
-                }
-            } else {
-                q->windowHandle()->setTransientParent(nullptr);
-                q->windowHandle()->setParent(nullptr);
-            }
-        }
-    }
-
     if (!newparent) {
         f |= Qt::Window;
         if (parent)
             targetScreen = q->parentWidget()->window()->screen();
     }
 
+    const bool destroyWindow = (
+        // Reparenting top level to child
+        (oldFlags & Qt::Window) && !(f & Qt::Window)
+        // And we can dispose of the window
+        && wasCreated && !q->testAttribute(Qt::WA_NativeWindow)
+    );
+
+    if (parent != newparent) {
+        // Update object parent now, so we can resolve new parent window below
+        QObjectPrivate::setParent_helper(newparent);
+
+        if (q->windowHandle())
+            q->windowHandle()->setFlags(f);
+
+        // If the widget itself or any of its children have been created,
+        // we need to reparent their QWindows as well.
+        QWidget *parentWithWindow = closestParentWidgetWithWindowHandle();
+        // But if the widget is about to be destroyed we must skip the
+        // widget itself, and only reparent children.
+        if (destroyWindow)
+            reparentWidgetWindowChildren(parentWithWindow);
+        else
+            reparentWidgetWindows(parentWithWindow, f);
+    }
+
     bool explicitlyHidden = isExplicitlyHidden();
 
-    // Reparenting toplevel to child
-    if (wasCreated && !(f & Qt::Window) && (oldFlags & Qt::Window) && !q->testAttribute(Qt::WA_NativeWindow)) {
+    if (destroyWindow) {
         if (extra && extra->hasWindowContainer)
             QWindowContainer::toplevelAboutToBeDestroyed(q);
 
-        QWindow *newParentWindow = newparent->windowHandle();
-        if (!newParentWindow)
-            if (QWidget *npw = newparent->nativeParentWidget())
-                newParentWindow = npw->windowHandle();
-
-        for (QObject *child : q->windowHandle()->children()) {
-            QWindow *childWindow = qobject_cast<QWindow *>(child);
-            if (!childWindow)
-                continue;
-
-            QWidgetWindow *childWW = qobject_cast<QWidgetWindow *>(childWindow);
-            QWidget *childWidget = childWW ? childWW->widget() : nullptr;
-            if (!childWW || (childWidget && childWidget->testAttribute(Qt::WA_NativeWindow)))
-                childWindow->setParent(newParentWindow);
+        // There shouldn't be any QWindow children left, but if there
+        // are, re-parent them now, before we destroy.
+        if (!q->windowHandle()->children().isEmpty()) {
+            QWidget *parentWithWindow = closestParentWidgetWithWindowHandle();
+            QWindow *newParentWindow = parentWithWindow ? parentWithWindow->windowHandle() : nullptr;
+            for (QObject *child : q->windowHandle()->children()) {
+                if (QWindow *childWindow = qobject_cast<QWindow *>(child)) {
+                    qCWarning(lcWidgetWindow) << "Reparenting" << childWindow
+                                              << "before destroying" << this;
+                    childWindow->setParent(newParentWindow);
+                }
+            }
         }
-        q->destroy();
+
+        // We have reparented any child windows of the widget we are
+        // about to destroy to the new parent window handle, so we can
+        // safely destroy this widget without destroying sub windows.
+        q->destroy(true, false);
     }
 
     adjustFlags(f, q);
@@ -11029,6 +11034,53 @@ void QWidgetPrivate::setParent_sys(QWidget *newparent, Qt::WindowFlags f)
     }
 }
 
+void QWidgetPrivate::reparentWidgetWindows(QWidget *parentWithWindow, Qt::WindowFlags windowFlags)
+{
+    if (QWindow *window = windowHandle()) {
+        // Reparent this QWindow, and all QWindow children will follow
+        if (parentWithWindow) {
+            // The reparented widget has not updated its window flags yet,
+            // so we can't ask the widget directly. And we can't use the
+            // QWindow flags, as unlike QWidgets the QWindow flags always
+            // reflect Qt::Window, even for child windows. And we can't use
+            // QWindow::isTopLevel() either, as that depends on the parent,
+            // which we are in the process of updating. So we propagate the
+            // new flags of the reparented window from setParent_sys().
+            if (windowFlags & Qt::Window) {
+                // Top level windows can only have transient parents,
+                // and the transient parent must be another top level.
+                QWidget *topLevel = parentWithWindow->window();
+                auto *transientParent = topLevel->windowHandle();
+                Q_ASSERT(transientParent);
+                qCDebug(lcWidgetWindow) << "Setting" << window << "transient parent to" << transientParent;
+                window->setTransientParent(transientParent);
+                window->setParent(nullptr);
+            } else {
+                auto *parentWindow = parentWithWindow->windowHandle();
+                qCDebug(lcWidgetWindow) << "Reparenting" << window << "into" << parentWindow;
+                window->setTransientParent(nullptr);
+                window->setParent(parentWindow);
+            }
+        } else {
+            qCDebug(lcWidgetWindow) << "Making" << window << "top level window";
+            window->setTransientParent(nullptr);
+            window->setParent(nullptr);
+        }
+    } else {
+        reparentWidgetWindowChildren(parentWithWindow);
+    }
+}
+
+void QWidgetPrivate::reparentWidgetWindowChildren(QWidget *parentWithWindow)
+{
+    for (auto *child : std::as_const(children)) {
+        if (auto *childWidget = qobject_cast<QWidget*>(child)) {
+            auto *childPrivate = QWidgetPrivate::get(childWidget);
+            childPrivate->reparentWidgetWindows(parentWithWindow);
+        }
+    }
+}
+
 /*!
     Scrolls the widget including its children \a dx pixels to the
     right and \a dy downward. Both \a dx and \a dy may be negative.