How do I create untitled slides and make it incremental for ioslides presentation with R Markdown

Question

I know that I can create untitled slides with ---, and use {.build} to make it incremental. But it doesn't work in combination... So as setting background using {data-background=a.png}.

If I simply use ## {.build}, or

{.build}
---

then the outline mess up in levels.

For slide with only a background I used the trick ##   {data-background=img/me1.png}. But ##   {.build} still leaves an empty space for a header.

So How can I make --- and {} both work?

Desired output:

1. Create a slide and show text, code, results, one by one.

2. No empty space at top saved for a header to have more vertical space

3. Clear level structure (indentation) in outline

Minimal, reproducible example:

---
title: "Untitled"
author: ""
date: ""
output: ioslides_presentation
---

# Section 1
## {.build}

This slide has no header to save vertical space

I know for incremental bullet I can add 

output: 
  ioslides_presentation:
    incremental: true

But I want to show text, code, plot one by one 

So I have to use {.build}


{.build}
---

Another method for no header incremental slide.

But now my outline got messed up in levels.

##   {data-background=a.png}

##   {.build}

For a slide with only a background 

I can use this `&nbsp` trick to keep my outline. 

But this still leaves an empty space for a header.

## Subsection 1

This is a future level 2 section slide. I want to show the structure with proper indentation in outline