Deduplicate lastWindowClosed handling

Whenever a window was closed, we had logic to check if it was the last
window, and if so emitted lastWindowClosed and then tried to quit the
application. But the automatic quit process also checked if there were
any remaining windows, as this process could be triggered as a result
of a QEventLoopLocker going out of scope. The two paths now share the
same logic for determining if there are any remaining windows.

The docs have been updated to reflect the original intent of the logic,
dealing only with primary windows (top levels without a transient parent).
This was how both the original code paths implemented their logic.

For historical reasons the Qt::WA_QuitOnClose attribute is closely
tied to the lastWindowClosed signal, and isn't merely limited to
controlling whether we try an automatic quit when the last window
closes. For now this behavior has been kept, but the docs have been
updated to clarify how the attribute is handled in practice.

Change-Id: If3d06b065236aad2e59e9091cac3146bc4cf79f6
Reviewed-by: Doris Verria <doris.verria@qt.io>
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>

src/corelib/global/qnamespace.qdoc

@@ -1126,10 +1126,10 @@
     e.g., when a hidden widget was resized. This flag is set or cleared by the
     Qt kernel.
 
-    \value WA_QuitOnClose Makes Qt quit the application when the last widget
-    with the attribute set has accepted closeEvent(). This behavior can be
-    modified with the QGuiApplication::quitOnLastWindowClosed property. By default
-    this attribute is set for all widgets of type Qt::Window.
+    \value WA_QuitOnClose Indicates that the widget should be taken into account
+    when deciding whether to quit the application when the last window is closed.
+    This behavior can be modified with the QGuiApplication::quitOnLastWindowClosed
+    property. By default this attribute is set for all widgets of type Qt::Window.
 
     \value WA_Resized Indicates that the widget has an explicit size. This flag
     is set or cleared by QWidget::resize() and QWidget::setGeometry().

src/gui/kernel/qguiapplication.cpp

@@ -1930,7 +1930,7 @@ bool QGuiApplication::notify(QObject *object, QEvent *event)
     bool accepted = QCoreApplication::notify(object, event);
 
     if (event->type() == QEvent::Close && object->isWindowType() && accepted)
-        d->maybeQuitOnLastWindowClosed(static_cast<QWindow*>(object));
+        d->maybeLastWindowClosed(static_cast<QWindow*>(object));
 
     return accepted;
 }
@@ -3530,7 +3530,8 @@ void QGuiApplicationPrivate::notifyWindowIconChanged()
     The default is \c true.
 
     If this property is \c true, the applications quits when the last visible
-    primary window (i.e. window with no parent) is closed.
+    \l{Primary and Secondary Windows}{primary window} (i.e. top level window
+    with no transient parent) is closed.
 
     \sa quit(), QWindow::close()
  */
@@ -3545,19 +3546,17 @@ bool QGuiApplication::quitOnLastWindowClosed()
     return QCoreApplication::isQuitLockEnabled();
 }
 
-void QGuiApplicationPrivate::maybeQuitOnLastWindowClosed(QWindow *closedWindow)
+void QGuiApplicationPrivate::maybeLastWindowClosed(QWindow *closedWindow)
 {
     Q_ASSERT(closedWindow);
 
-    if (!qt_window_private(closedWindow)->shouldTriggerQuitOnClose())
+    // Only windows that themselves participates in last-window-closed
+    // should be allowed to trigger lastWindowClosed.
+    if (!qt_window_private(closedWindow)->participatesInLastWindowClosed())
         return;
 
-    // Check if there are any remaining windows that should prevent us from quitting
-    for (auto *topLevelWindow : QGuiApplication::topLevelWindows()) {
-        auto *windowPrivate = qt_window_private(topLevelWindow);
-        if (windowPrivate->shouldCancelQuitOnClose())
-            return;
-    }
+    if (!lastWindowClosed())
+        return;
 
     if (in_exec)
         emit q_func()->lastWindowClosed();
@@ -3570,32 +3569,31 @@ void QGuiApplicationPrivate::maybeQuitOnLastWindowClosed(QWindow *closedWindow)
     \fn void QGuiApplication::lastWindowClosed()
 
     This signal is emitted from exec() when the last visible
-    primary window (i.e. window with no parent) is closed.
+    \l{Primary and Secondary Windows}{primary window} (i.e.
+    top level window with no transient parent) is closed.
 
     By default, QGuiApplication quits after this signal is emitted. This feature
     can be turned off by setting \l quitOnLastWindowClosed to \c false.
 
-    \sa QWindow::close(), QWindow::isTopLevel()
+    \sa QWindow::close(), QWindow::isTopLevel(), QWindow::transientParent()
 */
 
+bool QGuiApplicationPrivate::lastWindowClosed() const
+{
+    for (auto *window : QGuiApplication::topLevelWindows()) {
+        if (!qt_window_private(window)->participatesInLastWindowClosed())
+            continue;
+
+        if (window->isVisible())
+            return false;
+     }
+
+     return true;
+}
+
 bool QGuiApplicationPrivate::shouldQuit()
 {
-    const QWindowList processedWindows;
-    return shouldQuitInternal(processedWindows);
-}
-
-bool QGuiApplicationPrivate::shouldQuitInternal(const QWindowList &processedWindows)
-{
-    /* if there is no visible top-level window left, we allow the quit */
-    QWindowList list = QGuiApplication::topLevelWindows();
-    for (int i = 0; i < list.size(); ++i) {
-        QWindow *w = list.at(i);
-        if (processedWindows.contains(w))
-            continue;
-        if (w->isVisible() && !w->transientParent())
-            return false;
-    }
-    return true;
+    return lastWindowClosed();
 }
 
 void QGuiApplicationPrivate::quit()

src/gui/kernel/qguiapplication_p.h

@@ -108,8 +108,8 @@ public:
     virtual bool shouldQuit() override;
     void quit() override;
 
-    bool shouldQuitInternal(const QWindowList &processedWindows);
-    void maybeQuitOnLastWindowClosed(QWindow *closedWindow);
+    void maybeLastWindowClosed(QWindow *closedWindow);
+    bool lastWindowClosed() const;
 
     static void captureGlobalModifierState(QEvent *e);
     static Qt::KeyboardModifiers modifier_buttons;

src/gui/kernel/qwindow.cpp

@@ -2269,16 +2269,25 @@ bool QWindow::close()
     return d->platformWindow->close();
 }
 
-bool QWindowPrivate::shouldTriggerQuitOnClose() const
+bool QWindowPrivate::participatesInLastWindowClosed() const
 {
     Q_Q(const QWindow);
-    return q->isTopLevel();
-}
 
-bool QWindowPrivate::shouldCancelQuitOnClose() const
-{
-    Q_Q(const QWindow);
-    return q->isVisible() && !q->transientParent() && q->type() != Qt::ToolTip;
+    if (!q->isTopLevel())
+        return false;
+
+    // Tool-tip widgets do not normally have Qt::WA_QuitOnClose,
+    // but since we do not have a similar flag for non-widget
+    // windows we need an explicit exclusion here as well.
+    if (q->type() == Qt::ToolTip)
+        return false;
+
+    // A window with a transient parent is not a primary window,
+    // it's a secondary window.
+    if (q->transientParent())
+        return false;
+
+    return true;
 }
 
 /*!

src/gui/kernel/qwindow_p.h

@@ -118,8 +118,7 @@ public:
 
     virtual void processSafeAreaMarginsChanged() {}
 
-    virtual bool shouldTriggerQuitOnClose() const;
-    virtual bool shouldCancelQuitOnClose() const;
+    virtual bool participatesInLastWindowClosed() const;
 
     bool isPopup() const { return (windowFlags & Qt::WindowType_Mask) == Qt::Popup; }
     void setAutomaticPositionAndResizeEnabled(bool a)

src/widgets/kernel/qapplication.cpp

@@ -2614,24 +2614,6 @@ int QApplication::exec()
     return QGuiApplication::exec();
 }
 
-bool QApplicationPrivate::shouldQuit()
-{
-    /* if there is no non-withdrawn primary window left (except
-        the ones without QuitOnClose), we emit the lastWindowClosed
-        signal */
-    QWidgetList list = QApplication::topLevelWidgets();
-    QWindowList processedWindows;
-    for (int i = 0; i < list.size(); ++i) {
-        QWidget *w = list.at(i);
-        if (QWindow *window = w->windowHandle()) { // Menus, popup widgets may not have a QWindow
-            processedWindows.push_back(window);
-            if (w->isVisible() && !w->parentWidget() && w->testAttribute(Qt::WA_QuitOnClose))
-                return false;
-        }
-    }
-    return QGuiApplicationPrivate::shouldQuitInternal(processedWindows);
-}
-
 /*! \reimp
  */
 bool QApplication::notify(QObject *receiver, QEvent *e)
@@ -3301,7 +3283,7 @@ bool QApplication::notify(QObject *receiver, QEvent *e)
         res = d->notify_helper(receiver, e);
         // We don't call QGuiApplication::notify here, so we need to duplicate the logic
         if (res && e->type() == QEvent::Close)
-            d->maybeQuitOnLastWindowClosed(static_cast<QWindow *>(receiver));
+            d->maybeLastWindowClosed(static_cast<QWindow *>(receiver));
     } else {
         res = d->notify_helper(receiver, e);
     }

src/widgets/kernel/qapplication_p.h

@@ -106,8 +106,6 @@ public:
     virtual void notifyLayoutDirectionChange() override;
     virtual void notifyActiveWindowChange(QWindow *) override;
 
-    virtual bool shouldQuit() override;
-
     static bool autoSipEnabled;
     static QString desktopStyleKey();
 

src/widgets/kernel/qwidgetwindow.cpp

@@ -119,8 +119,7 @@ public:
             QWidgetPrivate::get(widget)->updateContentsRect();
     }
 
-    bool shouldTriggerQuitOnClose() const override;
-    bool shouldCancelQuitOnClose() const override;
+    bool participatesInLastWindowClosed() const override;
 };
 
 QRectF QWidgetWindowPrivate::closestAcceptableGeometry(const QRectF &rect) const
@@ -847,35 +846,18 @@ void QWidgetWindow::handleCloseEvent(QCloseEvent *event)
     event->setAccepted(accepted);
 }
 
-bool QWidgetWindowPrivate::shouldTriggerQuitOnClose() const
+bool QWidgetWindowPrivate::participatesInLastWindowClosed() const
 {
     Q_Q(const QWidgetWindow);
-    QWidget *widget = q->widget();
 
-    // Closing a window without WA_QuitOnClose should stop us from even
-    // looking at the other windows. Otherwise we might find that all the
-    // other windows miss WA_QuitOnClose as well, and end up quitting,
-    // but that's not what the user intended.
-    if (!widget->testAttribute(Qt::WA_QuitOnClose))
+    // For historical reasons WA_QuitOnClose has been closely tied
+    // to the lastWindowClosed signal, since the default behavior
+    // is to quit the application after emitting lastWindowClosed.
+    // ### Qt 7: Rename this attribute, or decouple behavior.
+    if (!q->widget()->testAttribute(Qt::WA_QuitOnClose))
         return false;
 
-    // Qt::Tool windows do not have WA_QuitOnClose set by default, which
-    // means that if you open a dialog from one, and the tool window is
-    // the only remaining window, then closing the dialog would result
-    // in quitting the application. To prevent this we check if the
-    // closed widget has a visible parent (the Qt:Tool window in this
-    // case), and if so bail out.
-    if (widget->parentWidget() && widget->parentWidget()->isVisible())
-        return false;
-
-    return true;
-}
-
-bool QWidgetWindowPrivate::shouldCancelQuitOnClose() const
-{
-    Q_Q(const QWidgetWindow);
-    QWidget *w = q->widget();
-    return w->isVisible() && !w->parentWidget() & w->testAttribute(Qt::WA_QuitOnClose);
+    return QWindowPrivate::participatesInLastWindowClosed();
 }
 
 #if QT_CONFIG(wheelevent)