Mouse control (drag) unreliable in Firefox

[tad-lispy]

Hey! I just watched your ElmConf talk and love the idea of Config UI. Thanks for sharing it.

It works great in Chromium, but in Firefox the drag is unreliable. When it doesn't work the following message is logged in the browser console:

Request for pointer lock was denied because Element.requestPointerLock() was not called from inside a short running user-generated event handler, and the document is not in full screen.

Sometimes it works, but mostly not. Here is a screen recording:

https://youtu.be/TKQRK8vzRgY