Since the dawn of time, V8's Array.p.sort has been implemented as a combination of quicksort (with sampling & recursive sorts to determine the pivot for large arrays) and insertion sort. 

Quicksort is not a stable sort algorithm, i.e. the order of equivalent elements ('equivalent' according to the given comparison function) is unspecified.

This sorting behavior has been baked into the web, and some sites may rely on its precise behavior.

Recent changes to Array.p.sort may break these assumptions. I'm opening this issue as a central tracking point.