From fdbb442badcc0a3b60631586814c5e3bd42f7ba7 Mon Sep 17 00:00:00 2001
From: Vicki Pfau <vi@endrift.com>
Date: Thu, 15 Apr 2021 19:00:02 -0700
Subject: [PATCH] Qt: Fix frames getting backlogged (fixes #2122)

---
 CHANGES                       |  1 +
 src/platform/qt/DisplayGL.cpp | 26 ++++++++++++++++++--------
 src/platform/qt/DisplayGL.h   |  2 +-
 3 files changed, 20 insertions(+), 9 deletions(-)

diff --git a/CHANGES b/CHANGES
index 118799f87ce..b7daf55391d 100644
--- a/CHANGES
+++ b/CHANGES
@@ -16,6 +16,7 @@ Other fixes:
  - Qt: Fix OpenGL renderer lagging behind when fast-forwarding (fixes mgba.io/i/2094)
  - Qt: Fix smudged window icon on Windows
  - Qt: Fix saving settings enabling camera when camera name changes (fixes mgba.io/i/2125)
+ - Qt: Fix frames getting backlogged (fixes mgba.io/i/2122)
 Misc:
  - Core: Truncate preloading ROMs that slightly exceed max size (fixes mgba.io/i/2093)
  - GBA: Default-enable VBA bug compat for Ruby and Emerald ROM hacks
diff --git a/src/platform/qt/DisplayGL.cpp b/src/platform/qt/DisplayGL.cpp
index c613e7afb61..de494cba313 100644
--- a/src/platform/qt/DisplayGL.cpp
+++ b/src/platform/qt/DisplayGL.cpp
@@ -371,7 +371,7 @@ void PainterGL::resizeContext() {
 	if (m_dims == size) {
 		return;
 	}
-	dequeueAll();
+	dequeueAll(false);
 	m_backend->setDimensions(m_backend, size.width(), size.height());
 }
 
@@ -429,7 +429,7 @@ void PainterGL::start() {
 }
 
 void PainterGL::draw() {
-	if (!m_active || m_queue.isEmpty()) {
+	if (!m_started || m_queue.isEmpty()) {
 		return;
 	}
 	mCoreSync* sync = &m_context->thread()->impl->sync;
@@ -461,6 +461,11 @@ void PainterGL::draw() {
 		performDraw();
 		m_backend->swap(m_backend);
 	}
+
+	QMutexLocker locker(&m_mutex);
+	if (!m_queue.isEmpty()) {
+		QTimer::singleShot(1, this, &PainterGL::draw);
+	}
 }
 
 void PainterGL::forceDraw() {
@@ -477,7 +482,7 @@ void PainterGL::forceDraw() {
 void PainterGL::stop() {
 	m_active = false;
 	m_started = false;
-	dequeueAll();
+	dequeueAll(false);
 	if (m_context) {
 		if (m_videoProxy) {
 			m_videoProxy->detach(m_context.get());
@@ -499,6 +504,7 @@ void PainterGL::stop() {
 
 void PainterGL::pause() {
 	m_active = false;
+	dequeueAll(true);
 }
 
 void PainterGL::unpause() {
@@ -551,20 +557,24 @@ void PainterGL::dequeue() {
 	m_buffer = buffer;
 }
 
-void PainterGL::dequeueAll() {
+void PainterGL::dequeueAll(bool keep) {
+	QMutexLocker locker(&m_mutex);
 	uint32_t* buffer = 0;
-	m_mutex.lock();
 	while (!m_queue.isEmpty()) {
 		buffer = m_queue.dequeue();
-		if (buffer) {
+		if (keep) {
+			if (m_buffer && buffer) {
+				m_free.append(m_buffer);
+				m_buffer = buffer;
+			}
+		} else if (buffer) {
 			m_free.append(buffer);
 		}
 	}
-	if (m_buffer) {
+	if (m_buffer && !keep) {
 		m_free.append(m_buffer);
 		m_buffer = nullptr;
 	}
-	m_mutex.unlock();
 }
 
 void PainterGL::setVideoProxy(std::shared_ptr<VideoProxy> proxy) {
diff --git a/src/platform/qt/DisplayGL.h b/src/platform/qt/DisplayGL.h
index d0cea76cb7e..e8951be9381 100644
--- a/src/platform/qt/DisplayGL.h
+++ b/src/platform/qt/DisplayGL.h
@@ -136,7 +136,7 @@ public slots:
 	void makeCurrent();
 	void performDraw();
 	void dequeue();
-	void dequeueAll();
+	void dequeueAll(bool keep = false);
 
 	std::array<std::array<uint32_t, 0x100000>, 3> m_buffers;
 	QList<uint32_t*> m_free;