It's a little weird in Firefox 63 in Linux. Holding down the mouse drags the image, but after I release, it changes the curve correctly until my mouse leaves the image. It also works fine if I hold down middle click.
This can be fixed by inserting this line after line 308 of euler_explorer.js, in the onpointerdown handler:
e.preventDefault();
By default, if you drag an image, then it moves the image. You should need to call preventDefault to prevent that, so I'm kind of annoyed with Chromium.
Argh, I used "modern web platform" but tried not to be too aggressive. I don't see any errors in the Safari console, so not sure what's wrong. The source is not (yet) in a public Git repo, but when it is, I'll happily accept PR's to improve browser compatibility.
Right, this is also why it doesn't work in Safari. That's fixed now, but imperfectly (I don't know how to do the equivalent of setPointerCapture in ancient browsers, and also it only adds a mouse, not a touch handler, so still doesn't work in mobile Safari). Thanks!
https://levien.com/euler_explorer/