Qt: Fix black screen when starting with a game (fixes #2781)

CHANGES

@@ -24,6 +24,7 @@ Other fixes:
  - Qt: Fix a handful of edge cases with graphics viewers (fixes mgba.io/i/2827)
  - Qt: Fix full-buffer rewind
  - Qt: Fix crash if loading a shader fails
+ - Qt: Fix black screen when starting with a game (fixes mgba.io/i/2781)
  - Scripting: Fix receiving packets for client sockets
  - Scripting: Fix empty receive calls returning unknown error on Windows
 Misc:

src/platform/qt/DisplayGL.cpp

@@ -48,6 +48,11 @@ using QOpenGLFunctions_Baseline = QOpenGLFunctions_3_2_Core;
 
 using namespace QGBA;
 
+enum ThreadStartFrom {
+	START = 1,
+	PROXY = 2,
+};
+
 QHash<QSurfaceFormat, bool> DisplayGL::s_supports;
 
 uint qHash(const QSurfaceFormat& format, uint seed) {
@@ -235,7 +240,16 @@ void DisplayGL::startDrawing(std::shared_ptr<CoreController> controller) {
 	messagePainter()->resize(size(), devicePixelRatio());
 #endif
 
-	CoreController::Interrupter interrupter(controller);
+	startThread(ThreadStartFrom::START);
+}
+
+void DisplayGL::startThread(int from) {
+	m_threadStartPending |= from;
+	if (m_threadStartPending < 3) {
+		return;
+	}
+
+	CoreController::Interrupter interrupter(m_context);
 	QMetaObject::invokeMethod(m_painter.get(), "start");
 	if (!m_gl) {
 		if (shouldDisableUpdates()) {
@@ -310,6 +324,7 @@ void DisplayGL::stopDrawing() {
 			hide();
 		}
 		setUpdatesEnabled(true);
+		m_threadStartPending &= ~1;
 	}
 	m_context.reset();
 }
@@ -435,6 +450,7 @@ void DisplayGL::setupProxyThread() {
 #if defined(_WIN32) && defined(USE_EPOXY)
 		epoxy_handle_external_wglMakeCurrent();
 #endif
+		QMetaObject::invokeMethod(this, "startThread", Q_ARG(int, ThreadStartFrom::PROXY));
 	});
 	connect(m_painter.get(), &PainterGL::texSwapped, m_proxyContext.get(), [this]() {
 		if (!m_context->hardwareAccelerated()) {

src/platform/qt/DisplayGL.h

@@ -113,6 +113,7 @@ protected:
 	virtual void resizeEvent(QResizeEvent*) override;
 
 private slots:
+	void startThread(int);
 	void setupProxyThread();
 
 private:
@@ -123,6 +124,7 @@ private:
 
 	bool m_isDrawing = false;
 	bool m_hasStarted = false;
+	int m_threadStartPending = 0;
 	std::unique_ptr<PainterGL> m_painter;
 	QThread m_drawThread;
 	QThread m_proxyThread;