diff --git a/rendering/renderfunctions.cpp b/rendering/renderfunctions.cpp
index d963ca962..9ccb3d827 100644
--- a/rendering/renderfunctions.cpp
+++ b/rendering/renderfunctions.cpp
@@ -541,10 +541,6 @@ GLuint compose_sequence(ComposeSequenceParams &params) {
 
 
 
-            // thread safety (see docs for ComposeSequenceParams::main_buffer_mutex)
-            if (final_fbo == params.main_buffer) {
-              params.main_buffer_mutex->lock();
-            }
 
             // bind front buffer as draw buffer
             params.ctx->functions()->glBindFramebuffer(GL_DRAW_FRAMEBUFFER, final_fbo);
@@ -593,10 +589,6 @@ GLuint compose_sequence(ComposeSequenceParams &params) {
             // unbind framebuffer
             params.ctx->functions()->glBindFramebuffer(GL_DRAW_FRAMEBUFFER, 0);
 
-            // thread safety (see docs for ComposeSequenceParams::main_buffer_mutex)
-            if (final_fbo == params.main_buffer) {
-              params.main_buffer_mutex->unlock();
-            }
 
 
 
diff --git a/rendering/renderfunctions.h b/rendering/renderfunctions.h
index 0decab71d..f733c560a 100644
--- a/rendering/renderfunctions.h
+++ b/rendering/renderfunctions.h
@@ -174,16 +174,6 @@ struct ComposeSequenceParams {
      */
     GLuint main_attachment;
 
-    /**
-     * @brief Mutex for the main framebuffer
-     *
-     * Used only for video rendering. Never accessed with audio rendering.
-     *
-     * If this is not nullptr, compose_sequence() will lock this mutex when rendering to main_buffer/main_attachment.
-     * Used to synchronize ViewerWidget and ViewerWindow with RenderThread.
-     */
-    QMutex* main_buffer_mutex;
-
     /**
      * @brief Backend OpenGL framebuffer 1 used for further processing before rendering to main_buffer
      *
diff --git a/rendering/renderthread.cpp b/rendering/renderthread.cpp
index eb421e2fa..62dbb2c83 100644
--- a/rendering/renderthread.cpp
+++ b/rendering/renderthread.cpp
@@ -203,7 +203,9 @@ void RenderThread::paint() {
   params.backend_attachment2 = back_texture_2;
   params.main_buffer = front_buffer_switcher ? front_buffer1 : front_buffer2;
   params.main_attachment = front_buffer_switcher ? front_texture1 : front_texture2;
-  params.main_buffer_mutex = front_buffer_switcher ? &front_mutex1 : &front_mutex2;
+
+  QMutex& active_mutex = front_buffer_switcher ? front_mutex1 : front_mutex2;
+  active_mutex.lock();
 
   // bind framebuffer for drawing
   ctx->functions()->glBindFramebuffer(GL_DRAW_FRAMEBUFFER, params.main_buffer);
@@ -223,8 +225,13 @@ void RenderThread::paint() {
 
   compose_sequence(params);
 
+  // flush changes
+  ctx->functions()->glFinish();
+
   texture_failed = params.texture_failed;
 
+  active_mutex.unlock();
+
   if (!save_fn.isEmpty()) {
     if (texture_failed) {
       // texture failed, try again
@@ -263,9 +270,6 @@ void RenderThread::paint() {
   glDisable(GL_BLEND);
   glDisable(GL_TEXTURE_2D);
 
-  // flush changes
-  ctx->functions()->glFinish();
-
   // release
   ctx->functions()->glBindFramebuffer(GL_DRAW_FRAMEBUFFER, 0);
 }
diff --git a/ui/viewerwidget.cpp b/ui/viewerwidget.cpp
index 66b81f51c..4969ec931 100644
--- a/ui/viewerwidget.cpp
+++ b/ui/viewerwidget.cpp
@@ -613,6 +613,7 @@ void ViewerWidget::paintGL() {
     }
 
     glDisable(GL_TEXTURE_2D);
+    glFinish();
 
     if (window->isVisible()) {
       window->set_texture(tex, double(viewer->seq->width)/double(viewer->seq->height), tex_lock);