Oh. Videos… I’m bandwidth constrained.
However, I’ve noticed this problem on Windows 8.1 and 10; all browsers I’ve tried (Firefox, Chrome, Opera). I just tried it in Chrome’s private mode (no extensions, etc), same result. It’s something about the _computer_ (and I have a work computer using the same Chrome profile, so all the same extensions, and it works perfectly).
My guess above was that it was the HD resolution monitor; but obviously that’s not it. Perhaps it’s the presence of an integrated trackpad, or the Dell drivers for it.