diff --git a/src/plugins/platforms/cocoa/qcocoaeventdispatcher.h b/src/plugins/platforms/cocoa/qcocoaeventdispatcher.h
index 787b23ec4db..d501cb9400a 100644
--- a/src/plugins/platforms/cocoa/qcocoaeventdispatcher.h
+++ b/src/plugins/platforms/cocoa/qcocoaeventdispatcher.h
@@ -59,6 +59,8 @@
 
 #include <CoreFoundation/CoreFoundation.h>
 
+Q_FORWARD_DECLARE_OBJC_CLASS(NSWindow);
+
 QT_BEGIN_NAMESPACE
 
 Q_DECLARE_LOGGING_CATEGORY(lcEventDispatcher);
@@ -67,7 +69,7 @@ typedef struct _NSModalSession *NSModalSession;
 typedef struct _QCocoaModalSessionInfo {
     QPointer<QWindow> window;
     NSModalSession session;
-    void *nswindow;
+    NSWindow *nswindow;
 } QCocoaModalSessionInfo;
 
 class QCocoaEventDispatcherPrivate;
diff --git a/src/plugins/platforms/cocoa/qcocoaeventdispatcher.mm b/src/plugins/platforms/cocoa/qcocoaeventdispatcher.mm
index fd0a4b47172..66f2a7a21d0 100644
--- a/src/plugins/platforms/cocoa/qcocoaeventdispatcher.mm
+++ b/src/plugins/platforms/cocoa/qcocoaeventdispatcher.mm
@@ -374,6 +374,7 @@ bool QCocoaEventDispatcher::processEvents(QEventLoop::ProcessEventsFlags flags)
             // [NSApp run], which is the normal code path for cocoa applications.
             if (NSModalSession session = d->currentModalSession()) {
                 QBoolBlocker execGuard(d->currentExecIsNSAppRun, false);
+                qCDebug(lcEventDispatcher) << "Running modal session" << session;
                 while ([NSApp runModalSession:session] == NSModalResponseContinue && !d->interrupt) {
                     qt_mac_waitForMoreEvents(NSModalPanelRunLoopMode);
                     if (session != d->currentModalSessionCached) {
@@ -417,6 +418,7 @@ bool QCocoaEventDispatcher::processEvents(QEventLoop::ProcessEventsFlags flags)
                     // to use cocoa's native way of running modal sessions:
                     if (flags & QEventLoop::WaitForMoreEvents)
                         qt_mac_waitForMoreEvents(NSModalPanelRunLoopMode);
+                    qCDebug(lcEventDispatcher) << "Running modal session" << session;
                     NSInteger status = [NSApp runModalSession:session];
                     if (status != NSModalResponseContinue && session == d->currentModalSessionCached) {
                         // INVARIANT: Someone called [NSApp stopModal:] from outside the event
@@ -617,6 +619,8 @@ void QCocoaEventDispatcherPrivate::temporarilyStopAllModalSessions()
     for (int i=0; i<stackSize; ++i) {
         QCocoaModalSessionInfo &info = cocoaModalSessionStack[i];
         if (info.session) {
+            qCDebug(lcEventDispatcher) << "Temporarily ending modal session" << info.session
+                                       << "for" << info.nswindow;
             [NSApp endModalSession:info.session];
             info.session = nullptr;
             [(NSWindow*) info.nswindow release];
@@ -656,6 +660,8 @@ NSModalSession QCocoaEventDispatcherPrivate::currentModalSession()
             [(NSWindow*) info.nswindow retain];
             QRect rect = cocoaWindow->geometry();
             info.session = [NSApp beginModalSessionForWindow:nswindow];
+            qCDebug(lcEventDispatcher) << "Begun modal session" << info.session
+                                       << "for" << nswindow;
 
             // The call to beginModalSessionForWindow above processes events and may
             // have deleted or destroyed the window. Check if it's still valid.
@@ -704,6 +710,8 @@ void QCocoaEventDispatcherPrivate::cleanupModalSessions()
         currentModalSessionCached = nullptr;
         if (info.session) {
             Q_ASSERT(info.nswindow);
+            qCDebug(lcEventDispatcher) << "Ending modal session" << info.session
+                                       << "for" << info.nswindow;
             [NSApp endModalSession:info.session];
             [(NSWindow *)info.nswindow release];
         }
@@ -716,6 +724,14 @@ void QCocoaEventDispatcherPrivate::cleanupModalSessions()
 
 void QCocoaEventDispatcherPrivate::beginModalSession(QWindow *window)
 {
+    qCDebug(lcEventDispatcher) << "Adding modal session for" << window;
+
+    if (std::any_of(cocoaModalSessionStack.constBegin(), cocoaModalSessionStack.constEnd(),
+        [&](const auto &sessionInfo) { return sessionInfo.window == window; })) {
+        qCWarning(lcEventDispatcher) << "Modal session for" << window << "already exists!";
+        return;
+    }
+
     // We need to start spinning the modal session. Usually this is done with
     // QDialog::exec() for Qt Widgets based applications, but for others that
     // just call show(), we need to interrupt().
@@ -736,6 +752,8 @@ void QCocoaEventDispatcherPrivate::beginModalSession(QWindow *window)
 
 void QCocoaEventDispatcherPrivate::endModalSession(QWindow *window)
 {
+    qCDebug(lcEventDispatcher) << "Removing modal session for" << window;
+
     Q_Q(QCocoaEventDispatcher);
 
     // Mark all sessions attached to window as pending to be stopped. We do this
@@ -966,6 +984,8 @@ QCocoaEventDispatcher::~QCocoaEventDispatcher()
     for (int i = 0; i < d->cocoaModalSessionStack.count(); ++i) {
         QCocoaModalSessionInfo &info = d->cocoaModalSessionStack[i];
         if (info.session) {
+            qCDebug(lcEventDispatcher) << "Ending modal session" << info.session
+                                       << "for" << info.nswindow << "during shutdown";
             [NSApp endModalSession:info.session];
             [(NSWindow *)info.nswindow release];
         }