The V3 approach obliterates this race condition by hooking addSourceBuffer at the MediaSource.prototype level, I intercept the creation of every SourceBuffer. The moment a buffer is created and returned, I immediately install a hooked appendBuffer directly on that specific instance; before any page code can even see the instance, let alone cache a reference to its methods. The hooked appendBuffer is installed as an own property of the instance, which takes precedence over the prototype chain. There is no window for fermaw to cache the original. The hook is always first.
Finding these optimization opportunities can itself be a significant undertaking. It requires end-to-end understanding of the spec to identify which behaviors are observable and which can safely be elided. Even then, whether a given optimization is actually spec-compliant is often unclear. Implementers must make judgment calls about which semantics they can relax without breaking compatibility. This puts enormous pressure on runtime teams to become spec experts just to achieve acceptable performance.
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