Don't show bogus empty window when calling QMainWindow::restoreState()

Qt doesn't create the actual QDockWidgets when restoring, the user must
ensure they are created before restoring state. So lets not create an empty
QDockWidgetGroupWindow which you can't close and with no tabs.

Change-Id: If0a6aa7cf6f3932ff4274e03f787e27aef8fa53d
Task-Id: QTBUG-57492
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>
Reviewed-by: Olivier Goffart (Woboq GmbH) <ogoffart@woboq.com>

1 files changed, 9 insertions, 2 deletions

diff --git a/src/widgets/widgets/qmainwindowlayout.cpp b/src/widgets/widgets/qmainwindowlayout.cpp
index aef8b9cbd5..14d7f3d835 100644
--- a/src/widgets/widgets/qmainwindowlayout.cpp
+++ b/src/widgets/widgets/qmainwindowlayout.cpp
@@ -1030,12 +1030,19 @@ bool QMainWindowLayoutState::restoreState(QDataStream &_stream,
                                                                  Qt::Horizontal, QTabBar::RoundedSouth, mainWindow);
                 QRect geometry;
                 stream >> geometry;
-                if (!floatingTab->layoutInfo()->restoreState(stream, dockWidgets, false))
+                QDockAreaLayoutInfo *info = floatingTab->layoutInfo();
+                if (!info->restoreState(stream, dockWidgets, false))
                     return false;
                 geometry = QDockAreaLayout::constrainedRect(geometry, floatingTab);
                 floatingTab->move(geometry.topLeft());
                 floatingTab->resize(geometry.size());
-                floatingTab->show();
+
+                // Don't show an empty QDockWidgetGroupWindow if no dock widget is available yet.
+                // reparentWidgets() would be triggered by show(), so do it explicitly here.
+                if (info->onlyHasPlaceholders())
+                    info->reparentWidgets(floatingTab);
+                else
+                    floatingTab->show();
             }
             break;
 #endif // QT_NO_TABBAR