STR: 1. Visit https://popcorn.webmaker.org/templates/basic/?savedDataUrl=ted.json 2. Wait until loaded (should be a few seconds). 3. Press play. Expected: Normal execution of Popcorn Maker. Actual: Crash. Everything works as expected in Chrome and Firefox, but Safari has given us trouble here. WebKit nightly has a slight different manifestation of the same problem, which we've managed to exploit as we attempted to troubleshoot this crash in our system. The crash occurs here, https://github.com/mozilla/butter/blob/master/src/timeline/timebar.js#L19 `tracksContainer.container.getBoundingClientRect().width` where `container` is erroneously `undefined`. The block which defines the `container` property for that object exists here, https://github.com/mozilla/butter/blob/master/src/timeline/track-container.js#L394, and the actual variable referenced is initialized, here https://github.com/mozilla/butter/blob/master/src/timeline/track-container.js#L17. Some experiments uncovered that when the crash happens, the getter function is never executed, so the evaluation of `tracksContainer.container` must not be using it at all, and simple returning `undefined`. Moreover, changing `tracksContainer.container` to `tracksContainer['container']` works as expected, and using the `value` parameter to immediately specify the value of the `container` works as well (e.g. `Object.defineProperty(this, 'container', { value: _container });`). Any help reducing this problem is appreciated, since it's situated deep within our app. I'm happy to assist where necessary.