Windows: Fix dialog moving up after closing/reshowing

A resize event delivered after closing the platform window
was causing the stored frame margins to be cleared.
Bail out of QWidgetWindow::updateMargins() if the
platform window is null.

Pick-to: 5.15
Fixes: QTBUG-79147
Change-Id: Iebbc90c3cccafa209cd720baedf45affb3f3c2b8
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@qt.io>

2 files changed, 27 insertions, 1 deletions

diff --git a/src/widgets/kernel/qwidgetwindow.cpp b/src/widgets/kernel/qwidgetwindow.cpp
index ae1e461d7a..cdc358aca2 100644
--- a/src/widgets/kernel/qwidgetwindow.cpp
+++ b/src/widgets/kernel/qwidgetwindow.cpp
@@ -736,8 +736,12 @@ bool QWidgetWindow::updateSize()
 
 void QWidgetWindow::updateMargins()
 {
-    const QMargins margins = frameMargins();
+    // QTBUG-79147 (Windows): Bail out on resize events after closing a dialog
+    // and destroying the platform window which would clear the margins.
     QTLWExtra *te = m_widget->d_func()->topData();
+    if (te->window == nullptr || te->window->handle() == nullptr)
+        return;
+    const QMargins margins = frameMargins();
     te->posIncludesFrame= false;
     te->frameStrut.setCoords(margins.left(), margins.top(), margins.right(), margins.bottom());
     m_widget->data->fstrut_dirty = false;
diff --git a/tests/auto/widgets/dialogs/qdialog/tst_qdialog.cpp b/tests/auto/widgets/dialogs/qdialog/tst_qdialog.cpp
index 9ef64480d6..53c33255bf 100644
--- a/tests/auto/widgets/dialogs/qdialog/tst_qdialog.cpp
+++ b/tests/auto/widgets/dialogs/qdialog/tst_qdialog.cpp
@@ -80,6 +80,7 @@ private slots:
     void transientParent_data();
     void transientParent();
     void dialogInGraphicsView();
+    void keepPositionOnClose();
 };
 
 // Testing get/set functions
@@ -543,5 +544,26 @@ void tst_QDialog::dialogInGraphicsView()
     }
 }
 
+// QTBUG-79147 (Windows): Closing a dialog by clicking the 'X' in the title
+// bar would offset the dialog position when shown next time.
+void tst_QDialog::keepPositionOnClose()
+{
+    QDialog dialog;
+    dialog.setWindowTitle(QTest::currentTestFunction());
+    const QRect availableGeometry = QGuiApplication::primaryScreen()->availableGeometry();
+    dialog.resize(availableGeometry.size() / 4);
+    const QPoint pos = availableGeometry.topLeft() + QPoint(100, 100);
+    dialog.move(pos);
+    dialog.show();
+    QVERIFY(QTest::qWaitForWindowExposed(&dialog));
+    dialog.close();
+    dialog.windowHandle()->destroy(); // Emulate a click on close by destroying the window.
+    QTest::qWait(50);
+    dialog.show();
+    QVERIFY(QTest::qWaitForWindowExposed(&dialog));
+    QTest::qWait(50);
+    QCOMPARE(dialog.pos(), pos);
+}
+
 QTEST_MAIN(tst_QDialog)
 #include "tst_qdialog.moc"