robert-ancell · Sep 1, 2026 · Sep 1, 2026
@@ -113,6 +113,18 @@ void fl_opengl_frame_composite(FlOpenGLFrame* self,
                       fl_framebuffer_get_id(self->framebuffer));
     glReadPixels(0, 0, width, height, GL_RGBA, GL_UNSIGNED_BYTE, self->pixels);
     glBindFramebuffer(GL_READ_FRAMEBUFFER, saved_read_framebuffer_binding);
+  } else {
+    // This frame is drawn by GTK on the main thread using a different OpenGL
+    // context to the one it was rendered with. Sharing a texture between
+    // contexts requires the rendering to have completed before the other
+    // context uses it - glFlush() only guarantees the commands have been
+    // submitted, not that they have finished. Without this GTK can sample a
+    // partially rendered frame, and drivers can fail in ways that leave the
+    // context unusable.
+    //
+    // The frame is copied out of the GPU with glReadPixels() when it can't be
+    // shared, which already waits for the rendering to complete.
+    glFinish();
   }
 
   glBindFramebuffer(GL_DRAW_FRAMEBUFFER, saved_draw_framebuffer_binding);

@@ -136,3 +136,54 @@ TEST_F(FlOpenGLFrameTest, ZeroSizeClearsFrame) {
   EXPECT_EQ(frame_width, 0u);
   EXPECT_EQ(frame_height, 0u);
 }
+
+// Checks a shareable frame is synchronized before it is handed to GTK, which
+// draws it using a different OpenGL context.
+// https://github.com/flutter/flutter/issues/191775
+TEST_F(FlOpenGLFrameTest, ShareableFrameSynchronized) {
+  constexpr size_t width = 100;
+  constexpr size_t height = 100;
+
+  g_autoptr(FlOpenGLFrame) frame = fl_opengl_frame_new(/*shareable=*/TRUE);
+  g_autoptr(FlFramebuffer) framebuffer =
+      fl_framebuffer_new(GL_RGBA, width, height, FALSE);
+  FlutterBackingStore backing_store = {
+      .type = kFlutterBackingStoreTypeOpenGL,
+      .open_gl = {
+          .type = kFlutterOpenGLTargetTypeFramebuffer,
+          .framebuffer = {.target = GL_RGBA8, .user_data = framebuffer}}};
+  FlutterLayer layer = {.type = kFlutterLayerContentTypeBackingStore,
+                        .backing_store = &backing_store,
+                        .offset = {0, 0},
+                        .size = {width, height}};
+  const FlutterLayer* layers[1] = {&layer};
+
+  EXPECT_CALL(epoxy, glFinish());
+
+  fl_opengl_frame_composite(frame, compositor, layers, 1);
+}
+
+// Checks a frame that is copied into CPU memory does not pay for a second
+// synchronization - glReadPixels() already waits for the frame to be rendered.
+TEST_F(FlOpenGLFrameTest, UnshareableFrameNotSynchronizedTwice) {
+  constexpr size_t width = 100;
+  constexpr size_t height = 100;
+
+  g_autoptr(FlOpenGLFrame) frame = fl_opengl_frame_new(/*shareable=*/FALSE);
+  g_autoptr(FlFramebuffer) framebuffer =
+      fl_framebuffer_new(GL_RGBA, width, height, FALSE);
+  FlutterBackingStore backing_store = {
+      .type = kFlutterBackingStoreTypeOpenGL,
+      .open_gl = {
+          .type = kFlutterOpenGLTargetTypeFramebuffer,
+          .framebuffer = {.target = GL_RGBA8, .user_data = framebuffer}}};
+  FlutterLayer layer = {.type = kFlutterLayerContentTypeBackingStore,
+                        .backing_store = &backing_store,
+                        .offset = {0, 0},
+                        .size = {width, height}};
+  const FlutterLayer* layers[1] = {&layer};
+
+  EXPECT_CALL(epoxy, glFinish()).Times(0);
+
+  fl_opengl_frame_composite(frame, compositor, layers, 1);
+}
@@ -306,6 +306,13 @@ static void fl_view_renderer_subsurface_present_layers(
 
   glBindFramebuffer(GL_DRAW_FRAMEBUFFER, saved_draw_framebuffer_binding);
 
+  // The frame is presented below using the subsurface's own OpenGL context.
+  // Sharing a texture between contexts requires the rendering to have completed
+  // before the other context uses it - making a context current only flushes
+  // the previous one, which guarantees the commands have been submitted, not
+  // that they have finished.
+  glFinish();
+
   // Present the composited frame directly to the subsurface using its own EGL
   // context. This reads the engine's frame texture directly, as the subsurface
   // context shares resources with the engine.

@@ -506,6 +506,12 @@ void _glDeleteTextures(GLsizei n, const GLuint* textures) {
   }
 }
 
+void _glFinish() {
+  if (mock) {
+    mock->glFinish();
+  }
+}
+
 static void _glDisable(GLenum cap) {
   _setEnable(cap, GL_FALSE);
 }
@@ -807,6 +813,7 @@ GLuint (*epoxy_glCreateShader)(GLenum shaderType);
 void (*epoxy_glDeleteFramebuffers)(GLsizei n, const GLuint* framebuffers);
 void (*expoxy_glDeleteShader)(GLuint shader);
 void (*epoxy_glDeleteTextures)(GLsizei n, const GLuint* textures);
+void (*epoxy_glFinish)();
 void (*epoxy_glFramebufferRenderbuffer)(GLenum target,
                                         GLenum attachment,
                                         GLenum renderbuffertarget,
@@ -898,6 +905,7 @@ static void library_init() {
   epoxy_glDeleteRenderbuffers = _glDeleteRenderbuffers;
   epoxy_glDeleteShader = _glDeleteShader;
   epoxy_glDeleteTextures = _glDeleteTextures;
+  epoxy_glFinish = _glFinish;
   epoxy_glDisable = _glDisable;
   epoxy_glEnable = _glEnable;
   epoxy_glFramebufferRenderbuffer = _glFramebufferRenderbuffer;

@@ -49,6 +49,7 @@ class MockEpoxy {
               glDeleteRenderbuffers,
               (GLsizei n, const GLuint* renderbuffers));
   MOCK_METHOD(void, glDeleteTextures, (GLsizei n, const GLuint* textures));
+  MOCK_METHOD(void, glFinish, ());
   MOCK_METHOD(void, glGenFramebuffers, (GLsizei n, GLuint* framebuffers));
   MOCK_METHOD(void, glGenRenderbuffers, (GLsizei n, GLuint* renderbuffers));
   MOCK_METHOD(void, glGenTextures, (GLsizei n, GLuint* textures));