Prevent window "flashing" when closing the last session

There are two scenarios when closing a window:
A) clicking the close button on the title bar (or Ctrl+Shift+Q):
~MainWindow()
~ViewManager()
~TabbedViewContainer()
~TerminalDisplay()
~Session()

B) closing the last session/tab in a window:
SessionController::sessionFinished()
~Session()
~TerminalDisplay()
~TabbedViewContainer()
~MainWindow()
~ViewManager()

the issue with the second case is that the TerminalDisplay is torn down
first, which exposes the TabbedViewContainer widget, the latter has the same
Qt::Window colour as the system colour scheme window background colour, if
you're using a dark terminal colour scheme and a light-coloured system colour
scheme, you could see some "flashing" when you close the last session with
e.g. Ctrl+D.

To fix this, in sessionFinished() check if TabbedViewContainer::count() is
1 (i.e. closing last tab/session), and emit the empty() signal in that case,
which is connected to MainwWindow::close(), then the order of tear down
becomes:
SessionController::sessionFinished()
~Session()
~MainWindow()
~ViewManager()
~TabbedViewContainer()
~TerminalDisplay()

BUG: 432077
FIXED-IN: 21.12
(cherry picked from commit bbec72250d)

src/MainWindow.cpp

@@ -102,7 +102,7 @@ MainWindow::MainWindow() :
 
     // create view manager
     _viewManager = new ViewManager(this, actionCollection());
-    connect(_viewManager, &Konsole::ViewManager::empty, this, &Konsole::MainWindow::close);
+    connect(_viewManager, &Konsole::ViewManager::empty, this, &QWidget::close);
     connect(_viewManager, &Konsole::ViewManager::activeViewChanged, this,
             &Konsole::MainWindow::activeViewChanged);
     connect(_viewManager, &Konsole::ViewManager::unplugController, this,

src/ViewManager.cpp

@@ -461,6 +461,13 @@ void ViewManager::sessionFinished()
         return;
     }
 
+    // The last session/tab? emit empty() so that close() is called in
+    // MainWindow, fixes #432077
+    if (_viewContainer->count() == 1) {
+        Q_EMIT empty();
+        return;
+    }
+
     auto *session = qobject_cast<Session *>(sender());
     Q_ASSERT(session);