Please see my reproduction (https://github.com/alifeee/revealjs-rstretch-bug) on http://alifeee.co.uk/revealjs-rstretch-bug/

Importantly, here is the markup of the problem slide:

        <section data-auto-animate>
          <h1 data-id="h1"><code>class="r-stack r-stretch"</code></h1>
          <p>
            Stack only. On Firefox, limits section to viewport. On Chrome,
            stretches off bottom of viewport :(
          </p>
          <div class="r-stack r-stretch">
            <img src="images/wide.png" alt="wide image" />
            <img src="images/tall.png" alt="tall image" />
          </div>
        </section>

Firefox

All is good

Chrome

All is not good (images stretch off the bottom)

Why?

I am not sure why. It seems a difference in how Firefox and Chrome deal with grid layout?

See the grid overlay here, and I have highlighted the div containing the images

Firefox

Div is sized correctly, and grid layout is the same size

Chrome

Div is sized correctly, but the grid layout overflows past this size

Research

After doing some research, I think I have found a fix. I will create a PR. I will also create this issue for people to find.