Windows: Destroy display change window before unregistering window class

The QWindowsScreenManager's m_displayChangeObserver window would still
be alive at the time ~QWindowsContext would unregister window classes,
resulting in a warning that was hidden by the default verbosity of
QWindowsContext::verbose. The result was that we would get a warning
when then trying to re-register the already registered window class.

We now do what we do for m_powerNotification and m_powerDummyWindow,
which is to destroy them explicitly before unregistering their class.

Fixes: QTBUG-125058
Pick-to: 6.5
Change-Id: Id3b02237fdd81f734a58a2314a157cad1289a82c
Reviewed-by: Oliver Wolff <oliver.wolff@qt.io>
(cherry picked from commit 48cc43a1e00e564438227dc0c251c988cee22483)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/plugins/platforms/windows/qwindowscontext.cpp

@@ -204,6 +204,8 @@ QWindowsContext::~QWindowsContext()
     if (d->m_powerDummyWindow)
         DestroyWindow(d->m_powerDummyWindow);
 
+    d->m_screenManager.destroyWindow();
+
     unregisterWindowClasses();
     if (d->m_oleInitializeResult == S_OK || d->m_oleInitializeResult == S_FALSE) {
 #ifdef QT_USE_FACTORY_CACHE_REGISTRATION

src/plugins/platforms/windows/qwindowsscreen.cpp

@@ -698,11 +698,15 @@ void QWindowsScreenManager::initialize()
     handleScreenChanges();
 }
 
-QWindowsScreenManager::~QWindowsScreenManager()
+void QWindowsScreenManager::destroyWindow()
 {
+    qCDebug(lcQpaScreen) << "Destroying display change observer" << m_displayChangeObserver;
     DestroyWindow(m_displayChangeObserver);
+    m_displayChangeObserver = nullptr;
 }
 
+QWindowsScreenManager::~QWindowsScreenManager() = default;
+
 bool QWindowsScreenManager::isSingleScreen()
 {
     return QWindowsContext::instance()->screenManager().screens().size() < 2;

src/plugins/platforms/windows/qwindowsscreen.h

@@ -105,6 +105,7 @@ public:
 
     QWindowsScreenManager();
     void initialize();
+    void destroyWindow();
     ~QWindowsScreenManager();
 
     void clearScreens();