class AppBase : public AppBase_Internal
{
public:
    AppBase() : AppBase_Internal(this) {}
    virtual void imGuiDebug() {}
    virtual void prepareGui(Window *window) {}
    virtual void destroyGui() {}
    void buildFontAtlasIfNeeded() {
        _engine->updateFontGroupsIfNeeded(); // <-- This is the FontGroups I'm talking about in the screenshot.
    }
    virtual void beforeFrameRender() {}
    int main(const char *title, const ivec2 &windowSize, const char *projectName);
};

class App : public AppBase
{
    ...
    void beforeFrameRender() // <-- This is called every frame
    {
        _engine->getDefaultFontGroup()->setScale(_debug_resolutionScale);
        buildFontAtlasIfNeeded();
    }
}

void my_engine::AppBase_Internal::buildFontAtlasIfNeeded_ImGui(ImGui_ImplVulkanH_Window* wd)
{
    if (_engine->_isFontAtlasBuilt_ImGui) {
        return;
    }
    _engine->_isFontAtlasBuilt_ImGui = true;

    VkResult err;

    // [Originated from "example_sdl_vulkan > main.cpp" in official ImGui v1.84]
    // Load Fonts
    // - If no fonts are loaded, dear imgui will use the default font. You can also load multiple fonts and use ImGui::PushFont()/PopFont() to select them.
    // - AddFontFromFileTTF() will return the ImFont* so you can store it if you need to select the font among multiple.
    // - If the file cannot be loaded, the function will return NULL. Please handle those errors in your application (e.g. use an assertion, or display an error and quit).
    // - The fonts will be rasterized at a given size (w/ oversampling) and stored into a texture when calling ImFontAtlas::Build()/GetTexDataAsXXXX(), which ImGui_ImplXXXX_NewFrame below will call.
    // - Read 'docs/FONTS.md' for more instructions and details.
    // - Remember that in C/C++ if you want to include a backslash \ in a string literal you need to write a double backslash \\ !
    //io.Fonts->AddFontDefault();
    //io.Fonts->AddFontFromFileTTF("../../misc/fonts/Roboto-Medium.ttf", 16.0f);
    //io.Fonts->AddFontFromFileTTF("../../misc/fonts/Cousine-Regular.ttf", 15.0f);
    //io.Fonts->AddFontFromFileTTF("../../misc/fonts/DroidSans.ttf", 16.0f);
    //io.Fonts->AddFontFromFileTTF("../../misc/fonts/ProggyTiny.ttf", 10.0f);
    //ImFont* font = io.Fonts->AddFontFromFileTTF("c:\\Windows\\Fonts\\ArialUni.ttf", 18.0f, NULL, io.Fonts->GetGlyphRangesJapanese());
    //IM_ASSERT(font != NULL);

    // Upload Fonts
    {
        // Use any command queue
        VkCommandPool command_pool = wd->Frames[wd->FrameIndex].CommandPool;
        VkCommandBuffer command_buffer = wd->Frames[wd->FrameIndex].CommandBuffer;

        err = vkResetCommandPool(g_Device, command_pool, 0);
        check_vk_result(err);
        VkCommandBufferBeginInfo begin_info = {};
        begin_info.sType = VK_STRUCTURE_TYPE_COMMAND_BUFFER_BEGIN_INFO;
        begin_info.flags |= VK_COMMAND_BUFFER_USAGE_ONE_TIME_SUBMIT_BIT;
        err = vkBeginCommandBuffer(command_buffer, &begin_info);
        check_vk_result(err);

        ImGui_ImplVulkan_CreateFontsTexture(command_buffer);

        VkSubmitInfo end_info = {};
        end_info.sType = VK_STRUCTURE_TYPE_SUBMIT_INFO;
        end_info.commandBufferCount = 1;
        end_info.pCommandBuffers = &command_buffer;
        err = vkEndCommandBuffer(command_buffer);
        check_vk_result(err);
        err = vkQueueSubmit(g_Queue, 1, &end_info, VK_NULL_HANDLE);
        check_vk_result(err);

        err = vkDeviceWaitIdle(g_Device);
        check_vk_result(err);
        ImGui_ImplVulkan_DestroyFontUploadObjects();
    }
}

bool ImGui_ImplVulkan_CreateFontsTexture(VkCommandBuffer command_buffer)
{
    ImGuiIO& io = ImGui::GetIO();
    ImGui_ImplVulkan_Data* bd = ImGui_ImplVulkan_GetBackendData();
    ImGui_ImplVulkan_InitInfo* v = &bd->VulkanInitInfo;

    unsigned char* pixels;
    int width, height;
    io.Fonts->GetTexDataAsRGBA32(&pixels, &width, &height);
    size_t upload_size = width * height * 4 * sizeof(char);

    VkResult err;

    //_/--------------------------------------------------\_
    // EDIT: The reason is for multiple updates of FontAtlas Texture by calling this function multiple
    // times and to avoid memory leak.
    if (bd->FontImage) {
        vkDestroyImage(v->Device, bd->FontImage, v->Allocator);
        bd->FontImage = nullptr;
    }
    if (bd->FontMemory) {
        vkFreeMemory(v->Device, bd->FontMemory, v->Allocator);
        bd->FontMemory = nullptr;
    }
    if (bd->FontView) {
        vkDestroyImageView(v->Device, bd->FontView, v->Allocator);
        bd->FontView = nullptr;
    }
    // \--------------------------------------------------/

    // Create the Image:
    { ... }
    // Create the Image View:
    { ... }
    // Update the Descriptor Set:
    { ... }
    // Create the Upload Buffer:
    { ... }
    // Upload to Buffer:
    { ... }
    // Copy to Image:
    { ... }
    // Store our identifier
    io.Fonts->SetTexID((ImTextureID)(intptr_t)bd->FontImage);
    return true;
}

void my_engine::Engine::FontGroup::setScale(float scale)
{
    int old_isize = _iSizePixels;
    _iSizePixels = int(scale * _engine->_iDefaultFontSize);
    assert(_iSizePixels >= 1);
    if (old_isize != _iSizePixels) {
        _engine->_areFontGroupsUpdated = false;
    }
}

void my_engine::Engine::updateFontGroupsIfNeeded()
{
    if (_areFontGroupsUpdated) {
        return;
    }
    _areFontGroupsUpdated = true;

    clearFonts();
    for (auto it : _fontGroups) {
        switch (it->getType()) {
        case FontGroupType_Default: it->_font = loadFont("arial", it->_iSizePixels); break;
        case FontGroupType_Script:  it->_font = loadFont("consola", it->_iSizePixels); break;
        default: assert(0);
        }
    }
    buildFontAtlas(); // Not just skipped if there is no change in the FontAtlas because clearFonts forces to rebuild the FontAtlas, but don't worry, the update function is rarely performed.
}