From 6b4ef1dccd8b2d5f1727081f3eb53a1c4493a44a Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?=C3=98ystein=20Walle?= <oystein.walle@zivid.com>
Date: Fri, 28 Mar 2025 14:27:17 +0100
Subject: [PATCH] Fix closing of windows when using the Qt backend

Notify the main GUI thread upon receiving a close event instead of when
the windows is destroyed. Additionally there was a logic error in in
GuiReceiver::isLastWindow() that is corrected.

Fixes #6479 and #20822
---
 modules/highgui/src/window_QT.cpp | 18 ++++++++++--------
 modules/highgui/src/window_QT.h   |  2 +-
 2 files changed, 11 insertions(+), 9 deletions(-)

diff --git a/modules/highgui/src/window_QT.cpp b/modules/highgui/src/window_QT.cpp
index c8957d9536..f7cd08bc88 100644
--- a/modules/highgui/src/window_QT.cpp
+++ b/modules/highgui/src/window_QT.cpp
@@ -859,7 +859,7 @@ void GuiReceiver::isLastWindow()
         delete guiMainThread;//delete global_control_panel too
         guiMainThread = NULL;
 
-        if (!doesExternalQAppExist)
+        if (doesExternalQAppExist)
         {
             qApp->quit();
         }
@@ -1739,13 +1739,6 @@ CvWindow::CvWindow(QString name, int arg2)
 }
 
 
-CvWindow::~CvWindow()
-{
-    if (guiMainThread)
-        guiMainThread->isLastWindow();
-}
-
-
 void CvWindow::setMouseCallBack(CvMouseCallback callback, void* param)
 {
     myView->setMouseCallBack(callback, param);
@@ -2259,6 +2252,15 @@ void CvWindow::keyPressEvent(QKeyEvent *evnt)
 }
 
 
+void CvWindow::closeEvent(QCloseEvent* evnt)
+{
+    QWidget::closeEvent(evnt);
+
+    if (guiMainThread)
+        guiMainThread->isLastWindow();
+}
+
+
 void CvWindow::icvLoadControlPanel()
 {
     QSettings settings("OpenCV2", QFileInfo(QApplication::applicationFilePath()).fileName() + " control panel");
diff --git a/modules/highgui/src/window_QT.h b/modules/highgui/src/window_QT.h
index b93b9ba597..1224ba9d32 100644
--- a/modules/highgui/src/window_QT.h
+++ b/modules/highgui/src/window_QT.h
@@ -298,7 +298,6 @@ class CvWindow : public CvWinModel
     Q_OBJECT
 public:
     CvWindow(QString arg2, int flag = CV_WINDOW_NORMAL);
-    ~CvWindow();
 
     void setMouseCallBack(CvMouseCallback m, void* param);
 
@@ -349,6 +348,7 @@ public:
 
 protected:
     virtual void keyPressEvent(QKeyEvent* event) CV_OVERRIDE;
+    virtual void closeEvent(QCloseEvent* event) CV_OVERRIDE;
 
 private: