Qt: Fix crash on shutdown with BP mode open

pcsx2-qt/MainWindow.cpp

@@ -1972,6 +1972,7 @@ void MainWindow::closeEvent(QCloseEvent* event)
 	// If there's no VM, we can just exit as normal.
 	if (!s_vm_valid || !m_display_created)
 	{
+		m_is_closing = true;
 		saveStateToConfig();
 		if (m_display_created)
 			g_emu_thread->stopFullscreenUI();
@@ -2214,6 +2215,13 @@ std::optional<WindowInfo> MainWindow::acquireRenderWindow(bool recreate_window,
 	if (surfaceless)
 		return WindowInfo();
 
+	// very low-chance race here, if the user starts the fullscreen UI, and immediately closes the window.
+	if (m_is_closing)
+	{
+		m_display_created = false;
+		return std::nullopt;
+	}
+
 	createDisplayWidget(fullscreen, render_to_main);
 
 	std::optional<WindowInfo> wi = m_display_widget->getWindowInfo();

pcsx2-qt/QtHost.cpp

@@ -183,13 +183,13 @@ void EmuThread::startFullscreenUI(bool fullscreen)
 
 	// this should just set the flag so it gets automatically started
 	ImGuiManager::InitializeFullscreenUI();
-	m_run_fullscreen_ui = true;
+	m_run_fullscreen_ui.store(true, std::memory_order_release);
 	m_is_rendering_to_main = shouldRenderToMain();
 	m_is_fullscreen = fullscreen;
 
 	if (!MTGS::WaitForOpen())
 	{
-		m_run_fullscreen_ui = false;
+		m_run_fullscreen_ui.store(false, std::memory_order_release);
 		return;
 	}
 
@@ -207,20 +207,21 @@ void EmuThread::stopFullscreenUI()
 		QMetaObject::invokeMethod(this, &EmuThread::stopFullscreenUI, Qt::QueuedConnection);
 
 		// wait until the host display is gone
-		while (!QtHost::IsVMValid() && MTGS::IsOpen())
+		// have to test the bool, because MTGS::IsOpen() goes false as soon as the close request happens.
+		while (m_run_fullscreen_ui.load(std::memory_order_acquire) || (!QtHost::IsVMValid() && MTGS::IsOpen()))
 			QApplication::processEvents(QEventLoop::ExcludeUserInputEvents, 1);
 
 		return;
 	}
 
-	if (m_run_fullscreen_ui)
+	if (MTGS::IsOpen() && !VMManager::HasValidVM())
+		MTGS::WaitForClose();
+
+	if (m_run_fullscreen_ui.load(std::memory_order_acquire))
 	{
-		m_run_fullscreen_ui = false;
+		m_run_fullscreen_ui.store(false, std::memory_order_release);
 		emit onFullscreenUIStateChange(false);
 	}
-
-	if (MTGS::IsOpen() && !VMManager::HasValidVM())
-		MTGS::WaitForClose();
 }
 
 void EmuThread::startVM(std::shared_ptr<VMBootParameters> boot_params)

pcsx2-qt/QtHost.h

@@ -61,7 +61,7 @@ class EmuThread : public QThread
 	__fi bool isExclusiveFullscreen() const { return m_is_exclusive_fullscreen; }
 	__fi bool isRenderingToMain() const { return m_is_rendering_to_main; }
 	__fi bool isSurfaceless() const { return m_is_surfaceless; }
-	__fi bool isRunningFullscreenUI() const { return m_run_fullscreen_ui; }
+	__fi bool isRunningFullscreenUI() const { return m_run_fullscreen_ui.load(std::memory_order_acquire); }
 
 	__fi bool isOnEmuThread() const { return (QThread::currentThread() == this); }
 	__fi bool isOnUIThread() const { return (QThread::currentThread() == m_ui_thread); }
@@ -206,9 +206,9 @@ private Q_SLOTS:
 	QTimer* m_background_controller_polling_timer = nullptr;
 
 	std::atomic_bool m_shutdown_flag{false};
+	std::atomic_bool m_run_fullscreen_ui{false};
 
 	bool m_verbose_status = false;
-	bool m_run_fullscreen_ui = false;
 	bool m_is_rendering_to_main = false;
 	bool m_is_fullscreen = false;
 	bool m_is_exclusive_fullscreen = false;