Unexpected custom DOM wrapper property is undefined soon after it was assigned

[Timothy Hatcher]

TimelineRuler.js in the Web Inspector occasionally throws an exception and never records. The _labelElement property we add to a DOM node at the beginning of a loop body ends up being undefined at the end of the loop body. This smells like a JIT optimization bug to me since this code has always worked fine and only recently started happening. It is also very inconsistent on when it happens. Because it is so random when it happens, we haven't been able to pin point steps or make a reduction. Just resting the Inspector window, or zooming the Timeline in and out causes updateLayout to be called.

From the TimelineRuler updateLayout function:

        var dividerElement = this._headerElement.firstChild;
        for (var i = 0; i <= dividerCount; ++i) {
            if (!dividerElement) {
                dividerElement = document.createElement("div");
                dividerElement.className = WebInspector.TimelineRuler.DividerElementStyleClassName;
                this._headerElement.appendChild(dividerElement);

                var labelElement = document.createElement("div");
                labelElement.className = WebInspector.TimelineRuler.DividerLabelElementStyleClassName;
                dividerElement._labelElement = labelElement;
                dividerElement.appendChild(labelElement);
            }

            // ...

            var dividerTime = firstDividerTime + (sliceTime * i);

            // ...

            dividerElement._labelElement.textContent = isNaN(dividerTime) ? "" : Number.secondsToString(dividerTime - this._zeroTime, true);
            dividerElement = dividerElement.nextSibling;
        }

The exceptions happens on the second to last line in the loop. dividerElement._labelElement is undefined, but it should never be.

[Radar WebKit Bug Importer]

<rdar://problem/20702089>