Fix r-stack overflow behaviour on Chromium browsers

[alifeee]

closes #3597

For more context see #3597. See an example of the broken state and fixed state in http://alifeee.co.uk/revealjs-rstretch-bug/

Found this solution here: https://forum.freecodecamp.org/t/cross-browser-layout-issue/651656/6. See more discussion there, and the Chrome issue tracker here: https://issuetracker.google.com/issues/40944415

Other relevant readings:

• https://stackoverflow.com/questions/66116397/my-css-grid-layout-doesnt-fill-its-container-on-chrome-whats-causing-the-loss
• https://stackoverflow.com/questions/53965326/overflow-difference-between-chrome-and-firefox-in-grid-container
• https://stackoverflow.com/questions/59321050/css-grid-content-overflowing

[hakimel]

Thanks for the detailed issue report and fix!