Delete TitleBar buttons directly if they're not in their event handler

If they're inside the event handler, they will be deleted later, otherwise
immediately. By doing it immediately we prevent leaks at exit and make
valgrind and other tools happy. Specially since due to the
QTBUG-83030 workaround some leaks were introduced, as QTimers don't
run after qApp went away.

src/qtwidgets/views/TitleBar.cpp

@@ -28,6 +28,7 @@
 #include <QHBoxLayout>
 #include <QLabel>
 #include <QTimer>
+#include <QScopedValueRollback>
 
 using namespace KDDockWidgets;
 using namespace KDDockWidgets::QtWidgets;
@@ -102,6 +103,14 @@ QSize Button::sizeHint() const
     return QSize(m, m);
 }
 
+bool Button::event(QEvent *ev)
+{
+    // A Button can trigger the deletion of its parent, in which case we use deleteLater.
+    QScopedValueRollback<bool> guard(m_inEventHandler, true);
+
+    return QToolButton::event(ev);
+}
+
 class KDDockWidgets::QtWidgets::TitleBar::Private
 {
 public:
@@ -144,6 +153,13 @@ TitleBar::~TitleBar()
         if (!button)
             continue;
 
+        if (auto kddwButton = qobject_cast<Button *>(button); !kddwButton->m_inEventHandler) {
+            // Minor optimization. If the button is not in an event handler it's safe to delete immediately.
+            // This saves us from memory leaks at shutdown when using the below QTimer::singleShot() hack.
+            delete kddwButton;
+            continue;
+        }
+
         button->setParent(nullptr);
         if (usesQTBUG83030Workaround()) {
             QTimer::singleShot(0, button, [button] {

src/qtwidgets/views/TitleBar.h

@@ -110,8 +110,13 @@ class Button : public QToolButton
     ~Button() override;
 
 protected:
+    friend class QtWidgets::TitleBar;
+
+    bool event(QEvent *ev) override;
     QSize sizeHint() const override;
     void paintEvent(QPaintEvent *) override;
+
+    bool m_inEventHandler = false;
 };
 
 }