Allow for rearranging tabified QDockWidgets

Allow to drag and drop tabs

[ChangeLog][QtWidgets][Important Behavior Changes] The tabs
for the tabified docks can be moved by the user.

Task-number: QTBUG-1049
Task-number: QTBUG-2295
Task-number: QTBUG-4532
Task-number: QTBUG-18883
Task-number: QTBUG-35148
Change-Id: I7ef9d4987db081654bd5d648e14370b3d381a720
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@theqtcompany.com>
Reviewed-by: Jørgen Lind <jorgen.lind@theqtcompany.com>
Reviewed-by: Paul Olav Tvete <paul.tvete@theqtcompany.com>

src/widgets/widgets/qdockarealayout.cpp

@@ -2244,6 +2244,22 @@ QRect QDockAreaLayoutInfo::tabContentRect() const
 
     return result;
 }
+
+int QDockAreaLayoutInfo::tabIndexToListIndex(int tabIndex) const
+{
+    Q_ASSERT(tabbed && tabBar);
+    quintptr data = qvariant_cast<quintptr>(tabBar->tabData(tabIndex));
+    for (int i = 0; i < item_list.count(); ++i) {
+        if (tabId(item_list.at(i)) == data)
+            return i;
+    }
+    return -1;
+}
+
+void QDockAreaLayoutInfo::moveTab(int from, int to)
+{
+    item_list.move(tabIndexToListIndex(from), tabIndexToListIndex(to));
+}
 #endif // QT_NO_TABBAR
 
 /******************************************************************************

src/widgets/widgets/qdockarealayout_p.h

@@ -208,6 +208,9 @@ public:
     QSize tabBarSizeHint() const;
 
     QSet<QTabBar*> usedTabBars() const;
+
+    int tabIndexToListIndex(int) const;
+    void moveTab(int from, int to);
 #endif // QT_NO_TABBAR
 };
 

src/widgets/widgets/qmainwindowlayout.cpp

@@ -1280,7 +1280,9 @@ QTabBar *QMainWindowLayout::getTabBar()
         result->setDrawBase(true);
         result->setElideMode(Qt::ElideRight);
         result->setDocumentMode(_documentMode);
+        result->setMovable(true);
         connect(result, SIGNAL(currentChanged(int)), this, SLOT(tabChanged()));
+        connect(result, &QTabBar::tabMoved, this, &QMainWindowLayout::tabMoved);
     }
 
     usedTabBars.insert(result);
@@ -1316,6 +1318,16 @@ void QMainWindowLayout::tabChanged()
     if (QWidget *w = centralWidget())
         w->raise();
 }
+
+void QMainWindowLayout::tabMoved(int from, int to)
+{
+    QTabBar *tb = qobject_cast<QTabBar*>(sender());
+    Q_ASSERT(tb);
+    QDockAreaLayoutInfo *info = layoutState.dockAreaLayout.info(tb);
+    Q_ASSERT(info);
+
+    info->moveTab(from, to);
+}
 #endif // QT_NO_TABBAR
 
 bool QMainWindowLayout::startSeparatorMove(const QPoint &pos)

src/widgets/widgets/qmainwindowlayout_p.h

@@ -286,6 +286,7 @@ private Q_SLOTS:
 #ifndef QT_NO_DOCKWIDGET
 #ifndef QT_NO_TABBAR
     void tabChanged();
+    void tabMoved(int from, int to);
 #endif
 #endif
 private: