Cocoa integration - handle didResignKey correctly

When we receive didResignKey notification, before setting active window to
null we check that [NSApp keyWindow] is nil, but it's not. At the same moment,
[NSApp keyWindow] == windowNotification.object - so it's indeed the window that
resigned the key status, it's just NSApp keyWindow that is not nil yet.

Change-Id: Iedc85d806c7d583a2054dc4d7168af13c9c7a4e0
Task-number: QTBUG-49925
Reviewed-by: Morten Johan Sørvig <morten.sorvig@theqtcompany.com>

src/plugins/platforms/cocoa/qnsview.mm

@@ -423,7 +423,7 @@ static NSString *_q_NSWindowDidChangeOcclusionStateNotification = nil;
         // set the active window to zero here, the new key window's
         // NSWindowDidBecomeKeyNotification hander will change the active window
         NSWindow *keyWindow = [NSApp keyWindow];
-        if (!keyWindow) {
+        if (!keyWindow || keyWindow == windowNotification.object) {
             // no new key window, go ahead and set the active window to zero
             if (!m_platformWindow->windowIsPopupType() && !m_isMenuView)
                 QWindowSystemInterface::handleWindowActivated(0);