[Webkit-unassigned] [Bug 102819] New: Object properties set with `Object.defineProperty` disappear during execution

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=102819

           Summary: Object properties set with `Object.defineProperty`
                    disappear during execution
           Product: WebKit
           Version: 528+ (Nightly build)
          Platform: Unspecified
               URL: https://popcorn.webmaker.org/templates/basic/?savedDat
                    aUrl=ted.json
        OS/Version: Unspecified
            Status: UNCONFIRMED
          Severity: Normal
          Priority: P2
         Component: JavaScriptCore
        AssignedTo: webkit-unassigned at lists.webkit.org
        ReportedBy: bobby at mozillafoundation.org


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.

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