hides menu bar when the area isn't hovered / selected.

hideable_menubar.cpp

void menubar() {
    static float menubar_height = 30.0;

    const auto& io = ImGui::GetIO();
    const auto flags = ImGuiWindowFlags_NoDecoration | ImGuiWindowFlags_NoMove | ImGuiWindowFlags_AlwaysAutoResize;
    const auto window_pos = ImVec2(0, 0);
    const auto window_size = ImVec2(io.DisplaySize.x, menubar_height);
    ImGui::SetNextWindowPos(window_pos);
    ImGui::SetNextWindowSize(window_size);
    ImGui::SetNextWindowBgAlpha(0);

    ImGui::PushStyleVar(ImGuiStyleVar_WindowRounding, 0.0F);
    ImGui::PushStyleVar(ImGuiStyleVar_WindowBorderSize, 0.0F);
    ImGui::PushStyleVar(ImGuiStyleVar_WindowPadding, ImVec2(0,0));
    ImGui::PushStyleVar(ImGuiStyleVar_FramePadding, ImVec2(0,0));
    ImGui::PushStyleVar(ImGuiStyleVar_FrameRounding, 0.0F);
    ImGui::PushStyleVar(ImGuiStyleVar_FrameBorderSize, 0.0F);
    ImGui::PushStyleVar(ImGuiStyleVar_ItemSpacing, ImVec2(0,0));
    ImGui::PushStyleVar(ImGuiStyleVar_ItemInnerSpacing, ImVec2(0,0));
    ImGui::PushStyleVar(ImGuiStyleVar_IndentSpacing, 0.0F);
    ImGui::PushStyleVar(ImGuiStyleVar_CellPadding, ImVec2(0,0));
    ImGui::PushStyleVar(ImGuiStyleVar_TabRounding, 0.0F);

    ImGui::Begin("##menubar", nullptr, flags);
    {
        ImGui::PopStyleVar(11);
        if (ImGui::IsWindowFocused(ImGuiFocusedFlags_RootAndChildWindows) || ImGui::IsWindowHovered(ImGuiFocusedFlags_RootAndChildWindows)) {
            if (ImGui::BeginChild("##child", window_size, ImGuiChildFlags_None, flags | ImGuiWindowFlags_MenuBar)) {
                if (ImGui::BeginMenuBar()) {
                    if (ImGui::BeginMenu("File"))
                    {
                        menubar_tab_file(app);
                        ImGui::EndMenu();
                    }
                    if (ImGui::BeginMenu("Emulation", app->rom_loaded))
                    {
                        menubar_tab_emulation(app);
                        ImGui::EndMenu();
                    }
                    if (ImGui::BeginMenu("Options"))
                    {
                        menubar_tab_options(app);
                        ImGui::EndMenu();
                    }
                    if (ImGui::BeginMenu("Tools"))
                    {
                        menubar_tab_tools(app);
                        ImGui::EndMenu();
                    }
                    if (ImGui::BeginMenu("View"))
                    {
                        menubar_tab_view(app);
                        ImGui::EndMenu();
                    }
                    if (ImGui::BeginMenu("Help"))
                    {
                        menubar_tab_help(app);
                        ImGui::EndMenu();
                    }
                    ImGui::EndMenuBar();
                }
            }
            ImGui::EndChild();
        }
    }
    ImGui::End();
}