SDL_UpdateWindowSurface cover the result of SDL_GL_SwapWindow

Question

I'm trying to combine my cpu soft renderer with imgui. Before that I manage to render using SDL_UpdateWindowSurface by updating the pixels of the windowSurface. The code goes lie this:

void WindowApp::updateCanvas(uint8_t* pixels, int width, int height, int channel) {
        SDL_LockSurface(windowSurface);
        Uint32* surfacePixels = (Uint32*)windowSurface->pixels;
        for(int i=0;i<height;i++){
            for(int j=0;j<width;j++){
                int index = i*width + j;
                Uint32 _color = SDL_MapRGB(
                        windowSurface->format,
                        pixels[index * channel + 0],
                        pixels[index * channel + 1],
                        pixels[index * channel + 2]);
                surfacePixels[index] = _color;
            }
        }
        SDL_UnlockSurface(windowSurface);
        SDL_UpdateWindowSurface(window);
    }

I notice that imgui uses SDL_GL_SwapWindow to render the UI, but when I put these two functions together, It will only render the original result instead of the UI. It will render the UI when I delete the SDL_UpdateWindowSurface function. So how can I manage to render the imgui UI on top of my original result? The wrong code goes like this:

void WindowApp::updateCanvas(uint8_t* pixels, int width, int height, int channel) {
        // Start the Dear ImGui frame
        ImGui_ImplOpenGL3_NewFrame();
        ImGui_ImplSDL2_NewFrame();
        ImGui::NewFrame();

        static float f = 0.0f;
        static int counter = 0;

        ImGui::Begin("Hello, world!");                          // Create a window called "Hello, world!" and append into it.

        ImGui::Text("This is some useful text.");               // Display some text (you can use a format strings too)

        ImGui::SliderFloat("float", &f, 0.0f, 1.0f);            // Edit 1 float using a slider from 0.0f to 1.0f
        if (ImGui::Button("Button"))                            // Buttons return true when clicked (most widgets return true when edited/activated)
            counter++;
        ImGui::SameLine();
        ImGui::Text("counter = %d", counter);

        ImGui::Text("Application average %.3f ms/frame (%.1f FPS)", 1000.0f / ImGui::GetIO().Framerate, ImGui::GetIO().Framerate);
        ImGui::End();
        //SDL_LockSurface(windowSurface);
        Uint32* surfacePixels = (Uint32*)windowSurface->pixels;
        for(int i=0;i<height;i++){
            for(int j=0;j<width;j++){
                int index = i*width + j;
                Uint32 _color = SDL_MapRGB(
                        windowSurface->format,
                        pixels[index * channel + 0],
                        pixels[index * channel + 1],
                        pixels[index * channel + 2]);
                surfacePixels[index] = _color;
            }
        }

        //SDL_UnlockSurface(windowSurface);
        ImGui::Render();
        ImGui_ImplOpenGL3_RenderDrawData(ImGui::GetDrawData());
        SDL_UpdateWindowSurface(window);
        SDL_GL_SwapWindow(window);
    }