"ERROR 1282 in glViewport" when used in async/thread - c++

Question

I'm seeing "ERROR 1282 in glViewport" in my terminal whne I call glViewport from inside glfwSetFramebufferSizeCallback. Not sure why, I think it should be firing from the correct context, but I'm not sure. It worked fine before I tried to move my gameloop into a thread.
Here's the code, I tried to trim out the extraneous stuff:
const int winWidth = 800;
const int winHeight = 600;
absl::Status glfwMain() {
gl::setOpenGlVersion(4, 6, GLFW_OPENGL_CORE_PROFILE);
ASSIGN_OR_RETURN(GLFWwindow * window, gl::createWindow(winWidth, winHeight, "mario4ever"))
std::atomic<bool> breakLoop = false;
auto gameLoop = std::async(std::launch::async, [&window, &breakLoop]() {
glfwMakeContextCurrent(window);
if (!gladLoadGLLoader((GLADloadproc) glfwGetProcAddress)) {
return absl::InternalError("Failed to initialize GLAD");
}
/* snip */
glViewport(0, 0, winWidth, winHeight);
glfwSetFramebufferSizeCallback(window, [](GLFWwindow *window, int width, int height) {
std::cerr << "glfwSetFramebufferSizeCallback" << std::endl;
glViewport(0, 0, width, height); // <----- error fires here
});
while (!breakLoop) {
draw(window);
}
/* snip */
return absl::OkStatus();
});
while (!glfwWindowShouldClose(window)) {
processInput(window);
glfwPollEvents();
}
breakLoop = true;
return gameLoop.get();
return absl::OkStatus();
}
int main() {
glfwInit();
auto status = glfwMain();
glfwTerminate();
if (!status.ok()) {
std::cerr << status << std::endl;
}
return status.raw_code();
}
Whether I put the glfwSetFramebufferSizeCallback inside the std::async or outside doesn't seem to make a difference.
What's the problem?
This tutorial says error 1282 is GL_INVALID_VALUE and according to these docs that should happen only when "either width or height is negative".
I checked the width and height that the callback is giving me and neither is negative. They're about what I'd expect.
Tried adding a mutex:
std::mutex drawMutex;
void draw(GLFWwindow *window) {
std::lock_guard<std::mutex> lock(drawMutex);
glClear(GL_COLOR_BUFFER_BIT);
glDrawArrays(GL_TRIANGLES, 0, 3);
glfwSwapBuffers(window);
}
...
// inside `std::async` callback:
glfwSetFramebufferSizeCallback(window, [](GLFWwindow *window, int width, int height) {
std::lock_guard<std::mutex> lock(drawMutex);
glViewport(0, 0, width, height);
});