Demo web app not usable under Mac OS

[elektrobohemian]

When following the tutorial and after launching the local webserver with the provided mp4 file, further navigation becomes impossible. The arrow icons are inactive as well as the scroll bar.
Checked with Firefox 71, Opera 60.0.3255.109, and Chrome 79.

[jgonggrijp]

I tested this on MacOS too, but with Safari. I didn't run into this issue. I can do some testing if desired.