QDockWidget: Fix group unplugging

A floating dock widget could either be a single QDockWidget object,
or a QDockWidgetGroupWindow with a single QDockWidget child.
The former could be dropped on the latter. Dropping the latter on
the former caused a crash.

The existence of QDockWidgetGroupWindows with a single dock widget
child was accepted to be by design.
Previous fixes, such as  9ff40b59da58160dc26c54204a615a2456e07405,
attempted to wrap all single floating dock widgets in
QDockWidgetGroupWindows.

These attempts fell short, because of the manifold programmatic and
manual options to create a floating dock widget:
- drag a single dock widget out of a main window dock area
- drag a dock widget out of a tab bar on the main window
- drag a dock widget out of a floating tab
- call `QDockWidget::setFloating(true)` in any situation
- create a new QDockWidget, that floats from the beginning

Whenever a QDockWidgetGroupWindow with a single QDockWidget child
was hovered and/or dropped on a QDockWidget without a group window,
crashes or screen artifacts were observed. Previous fixes made them
occur less often.

QDockWidgetGroupWindow is not designed to hold a single QDockWidget
child. Such a state is inconsistent and may only exist, while a
QDockWidgetGroupWindow is constructed.

The reason why such invalid QDockWidgetGroupWindows started to exist,
is a bool trap: QDockWidgetPrivate::mouseMoveEvent() starts a drag
operation, when a dock widget is moved by mouse.
It called startDrag() with no argument, which defaulted to
startDrag(true) and caused a group drag.

This assumption is
*correct*, when a tabbed group of dock widgets is dragged out of the
main dock as a whole, to become floating tabs.
*wrong*, when a single dock widget is dragged out of a docked group,
to become a single floating dock widget.

In the second case, the dock widget was wrapped in a new, floating,
invisible QDockWidgetGroupWindow. Looking like a single, floating dock
widget, the group window caused a crash, when attempted to be dropped
on another dock widget.

This patch eliminates all cases, where a QDockWidgetGroupWindow with
a single QDockWidget is created:
(1) Implement QDockWidgetPrivate::isTabbed().
This enables mouseMoveEvent to determine, whether the move relates to a
group of tabbed dock widgets, or to a single dock widget.
startDrag() can therefore be called with the right argument. It will no
longer create a QDockWidgetGroupWindow with a single QDockWidget child.

(2) Change QMainWindowTabBar::mouseReleaseEvent
When a dock widget was dragged out of a tab bar and became a single,
floating dock widget, it was still parented to the group window.
That is wrong, because it has no more relationship with that group
window.
=> Reparent it to the main window, just like any other single floating
dock widget. That enables QDockWidgetGroupWindow to detect, that the
2nd last child has gone and no more group window is needed (see next
point).

(3) React to reparenting, closing and deleting
If the second last dock widget in a floating tab gets closed (manually
or programmatically), reparented or deleted, also unplug the last one
and remove the group window.

(4) Amend 9ff40b59da58160dc26c54204a615a2456e07405
Remove the code path where a QDockWidgetGroupWindow with a single
QDockWidget child was created 'just in case', to make it compatible
others, created by (1), (2) or (3).

(5) Change QMainWindowLayout::hover()
When the hover ends without a successful drop and a temporary group
window with a single dock widget child has been created, remove the
group window.

The patch fixes smaller inconsistencies, which have not become visible
due to assertions and crashes earlier in the chain.

The patch finally extends tst_QDockWidget, to cover all 4 cases.

- Creation of floating tabs
The creation of floating tabs is extracted from floatingTabs() to
the helper function createFloatingTabs(). In addition to creating
floating tabs, the helper verifies that dragging a dock widget out
of the main window doesn't accidently wrap it in a group window.
This covers case (1).

- tst_QDockWidget::floatingTabs()
The test function verifies now, that both test dock widgets have the
same path before plugging them together and after unplugging them from
the floating tab. This covers case(4).

- tst_QDockwidget::deleteFloatingTabWithSingleDockWidget()
This test function is added, to cover cases (2) and (3).

- tst_QDockWidget::hoverWithoutDrop()
This test function hovers two floating dock widgets hover each other,
and returns the moved dock widget to its origin before releasing the
mouse. This covers case(5).

This fixes a lot of long standing bugs, making the author of this patch
modestly happy :-)

Fixes: QTBUG-118223
Fixes: QTBUG-99136
Fixes: QTBUG-118578
Fixes: QTBUG-118579
Fixes: QTBUG-56799
Fixes: QTBUG-35736
Fixes: QTBUG-63448
Fixes: QTBUG-88329
Fixes: QTBUG-88157
Fixes: QTBUG-94097
Fixes: QTBUG-44540
Fixes: QTBUG-53808
Fixes: QTBUG-72915
Fixes: QTBUG-53438
Found-by: Keith Kyzivat <keith.kyzivat@qt.io>
Found-by: Frederic Lefebvre <frederic.lefebvre@qt.io>
Pick-to: 6.6 6.5
Change-Id: I51b5f9e40cb2dbe55fb14d769541067730538463
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>

1 files changed, 16 insertions, 1 deletions

diff --git a/src/widgets/widgets/qdockwidget.cpp b/src/widgets/widgets/qdockwidget.cpp
index ff4fda6bad..f76975859f 100644
--- a/src/widgets/widgets/qdockwidget.cpp
+++ b/src/widgets/widgets/qdockwidget.cpp
@@ -847,6 +847,11 @@ void QDockWidgetPrivate::endDrag(EndDragMode mode)
                         tabPosition = mwLayout->tabPosition(toDockWidgetArea(dwgw->layoutInfo()->dockPos));
                     }
 #endif
+                    // Reparent, if the drag was out of a dock widget group window
+                    if (mode == EndDragMode::LocationChange) {
+                        if (auto *groupWindow = qobject_cast<QDockWidgetGroupWindow *>(q->parentWidget()))
+                            groupWindow->reparent(q);
+                    }
                 }
                 q->activateWindow();
             } else {
@@ -948,6 +953,15 @@ bool QDockWidgetPrivate::mouseDoubleClickEvent(QMouseEvent *event)
     return false;
 }
 
+bool QDockWidgetPrivate::isTabbed() const
+{
+    Q_Q(const QDockWidget);
+    QDockWidget *that = const_cast<QDockWidget *>(q);
+    auto *mwLayout = qt_mainwindow_layout_from_dock(that);
+    Q_ASSERT(mwLayout);
+    return mwLayout->isDockWidgetTabbed(q);
+}
+
 bool QDockWidgetPrivate::mouseMoveEvent(QMouseEvent *event)
 {
     bool ret = false;
@@ -978,7 +992,8 @@ bool QDockWidgetPrivate::mouseMoveEvent(QMouseEvent *event)
             } else
 #endif
             {
-                startDrag(DragScope::Group);
+                const DragScope scope = isTabbed() ? DragScope::Group : DragScope::Widget;
+                startDrag(scope);
                 q->grabMouse();
                 ret = true;
             }